Abstract: Abstract: In the design of a greenhouse ventilation system, estimating the necessary ventilation rate is a prerequisite for determining the air volume and the number of fans, the size and the location of vents, and so on. However, in practice it is a complicated procedure to determine the necessary ventilation rate based on the 3 factors, i.e. temperature, humidity and CO2 (carbon dioxide) concentration. We face the following problems: (1) The absence of climate data; (2) Difficulties in choosing values for parameters, including solar heat gain correction factor, evapotranspiration coefficient, outdoor horizontal solar irradiance, outdoor design dry-bulb temperature, indoor design temperature, and so on. This paper aims to solve the problems above and enhance the practicality of the estimation method of the necessary ventilation rate, which is recommended in the edited version of NY/T 1451 Design Rule for Greenhouse Ventilation. Firstly, we determine the calculation method of necessary ventilation rate for capability design. Secondly, we calculate the monthly value of outdoor horizontal solar irradiance and outdoor design dry-bulb temperature of Chinese typical regions by referring to the American method of determining climatic design condition, and using available climate data in China. The monthly data are useful when there are seasonal variations in solar geometry and intensity, or greenhouse use patterns are taken into consideration. Lastly, this paper solves the problem of how to set values for solar heat gain correction factor, evapotranspiration coefficient, and indoor design temperature under different conditions. We compare the necessary ventilation rate estimated based on cooling air, dehumidification and enhancing CO2 concentration. The purpose of dehumidification is to prevent the plant disease caused by high humidity in greenhouse. High humidity usually occurs in the morning, when low ventilation is applied due to low temperature. The necessary ventilation rate obtained based on dehumidification is much smaller than that based on cooling air. The estimation based on enhancing CO2 concentration only guarantees the lower bound of CO2 concentration: If the CO2 concentration is no smaller than 300 μL/L, the estimated ventilation rate is 0.011 m3/(s·m2); when the CO2 concentration is close to the outside environment (400 μL/L), it tends to infinity. Therefore, the estimation based on CO2 concentration is not suitable for ventilation system design. Greenhouse solar heat gain correction factor shows the correlation between actual amount of solar radiation and greenhouse floor area, and is obtained based on solar altitude and greenhouse structural type. The value equals to the projected area of greenhouse divided by greenhouse floor area. For the common greenhouse structures in China, this parameter should be between 1.0 and 1.3 for gutter connected greenhouses, 1.0-1.1 for summer, and 1.2-1.3 for spring and autumn, and is negatively correlated to the size of gutter connected greenhouse. Alternatively, it should take a value between 1.0 and 1.5 for sunlight greenhouses (1.0-1.2 for summer, 1.3-1.5 for spring and autumn), and is positively correlated to the latitude in this case. The evapotranspiration coefficient is the ratio of the energy used to evaporate water from the canopy or water surface to the incoming solar energy. The existing research shows that this parameter is related to ventilation rate, leaf area index (LAI) and outside humidity ratio. However, at the project design stage, LAI is usually unknown. According to the worst case design, when plants are very young and sparse, LAI can take the value of 0.5; when plants are flourished and healthy, LAI is usually over 1.3. In China, outside humidity ratio in different areas varies, but their annual fluctuations follow certain patterns. Using data from China TMY2 (typical meteorological year), we can obtain the monthly maximum outside humidity ratios of 10 cities (Beijing, Shanghai, Haikou, Chongqing, Guangzhou, Shenyang, Harbin, Xi'an, Lanzhou and Urumchi). Between June and September, Urumchi has a monthly maximum outside humidity ratio of 9-16 g/kg, whereas the value in Haikou is 19-28 g/kg. As a result, based on outside humidity ratio and LAI we can estimate that, when plants are very young and sparse, the evapotranspiration coefficient should take value in 0.65-0.90; when plants are flourished and healthy, it should be in 0.80-1.15. Take a specific multi-span greenhouse in Beijing as the example, we use the method provided in this paper to estimate the necessary ventilation rate of young and flourished plants greenhouses respectively. We can see that when the climatic design conditions or the greenhouse functions are different, the engineering methods we adopt are different. In young plants greenhouse, due to a low LAI, energy used to evaporate water is small, and therefore, not only a large number of fans are requested, but evaporative pads should be used to cool air in May. However, in flourished plants greenhouses, cooling air by fans on its own is enough to meet the indoor temperature requirement. Our results suggest that the calculation method of necessary ventilation rate for capability design should be based on the factor of temperature. Our research provides a viable method of parameter estimation for this purpose.