Abstract: In order to solve the problem that the lack of relevant theories and specifications of capacity selection of greenhouse CO2 supply equipment, based on the model analysis of the constant concentration control of CO2, the low range concentration control of CO2, and the constant gas flow control of CO2, a CO2 balance model in greenhouse is established. There are very complex relationships between temperature, light intensity and CO2 concentration and photosynthesis intensity. To maintain a constant CO2 concentration in greenhouse, it is necessary to accurately regulate the supply of CO2 according to the changes in light and temperature. In addition, to maintain CO2 concentration above the saturation point, the constant concentration control of CO2 is also an uneconomical way. So this mode of regulation is difficult to implement in large-scale greenhouse production. The constant concentration control of CO2 is possible to be used only in experimental greenhouse which the light and temperature are accurately controlled. The low range concentration control of CO2 also needs to adjust the supply of CO2 frequently, therefore, the timeliness and accuracy requirements of CO2 supply source are high. It is generally applicable to liquefied CO2 supply system. The constant gas flow control of CO2 is the easiest to implement in engineering, as long as the gas supply valve is opened or closed according to the set time, and other control equipment can be omitted. Therefore, this control model is very suitable for the CO2 supply system which uses the recovered flue gas of the boiler as CO2 source, and also suitable for liquid CO2 supply system without considering economic factors. .Combined with crop demand for CO2 and the characteristics of low CO2 emission of soil in the large-scale gutter-connected greenhouse for vegetable spatial soilless culture, a calculation method for the CO2 supply load of equipment used for CO2 enrichment in greenhouse is proposed. Taking the recovered flue gas of a boiler and liquid CO2 as the CO2 sources, two CO2 supply methods are proposed. The calculation method of the capacity of gas boiler and the volume of liquid CO2 storage tank provide a theoretical basis for the design and selection of corresponding equipment. Taking a Venlo type greenhouse with the cultivation area of 49 200 m2 and the volume of 329 640 m3 as an example, the calculation method was checked. The calculation results showed that by using constant gas flow control of CO2, and under the condition that the concentration of CO2 was 600×10-6 m3/m3 in greenhouse, only one gas boiler with a rated evaporation capacity of 10t/h needed to be operated at the daytime to meet the demand of CO2 enrichment, when the recovered flue gas of the boiler was used as CO2 source. By using the low range concentration control of CO2, and under the condition that the concentration of CO2 was 500×10-6 m3/m3 in greenhouse, a storage tank with a volume of 20 m3 was needed for contain liquefied CO2 when using liquefied CO2 as the CO2 source. The measured CO2 concentration in greenhouse showed that the calculation method of CO2 supply load for equipment capacity selection established in this paper was feasible. The method proposed in this paper can be used for similar engineering design reference.