Abstract: Abstract: The closed greenhouse is a light-permeable greenhouse type, with totally-enclosed architectural structure. Cooling by ventilation is replaced completely by mechanical cooling. Excess solar energy is collected and stored to be reused to heat the greenhouse or other buildings. The closed greenhouse can achieve energy conservation and emission reduction, recycling water of evapotranspiration, maintaining a high level of CO2 concentration, as well as isolating the bacteria spores from external environment, etc. However, high air temperature inside the closed greenhouse is difficult to control effectively in summer, or a great deal of energy is needed to consume, resulting in a restriction of the closed greenhouse when used in actual production. In order to decrease air temperature inside the closed greenhouse, taking low carbon emission and energy-saving into consideration, a closed greenhouse with water-walls (CGWW) was designed and built in Changping District of Beijing, China. With an indoor ground surface of about 7.6 m2, it was supported by a steel skeleton and assembled from some glass tanks filled with water. And for suppressing the growth of green algae, pH value of the water was adjusted to 9.5. Water layer thickness of side walls and roof were 30 and 13 cm, respectively. Cooling characteristics of the CGWW in summer was tested from 26 Jul. to 10 Sep. 2015. During the test, cucumbers and rapes were cultivated inside the CGWW. The results showed that, average air temperature inside the CGWW was 29.4-34.3℃ around noon (10:00-16:00), decreased by 0.8-6.8℃ compared with ambient. Meanwhile, the air temperature drop range inside the CGWW got bigger with the increase of solar radiation or ambient air temperature (P＜0.01). In 94.6% of the photosynthesis period (06:00-18:00), air temperature inside the CGWW was controlled within 35℃, which could avoid the high temperature stress effectively. So the CGWW had remarkable effect for cooling in summer. During the nighttime, relative humidity inside the CGWW was controlled within 80%, and the average value was 54.7%-73.7%, decreased by 7.2%-17.5% compared with ambient. Meanwhile, there was a negative linear correlation between humidity difference and temperature difference, inside and outside the CGWW (P＜0.01). During the daytime, solar radiation in horizontal direction inside the CGWW was 31.5-67.4 W/m2, and accounted for 11.9%-17.8% of that outside the CGWW. As solar radiation transmitted into the CGWW from outside, ratio of far-red light decreased from 41.9% to 9.2%, with a transmittance of 6.0%, which was conducive to the suppression of high temperature inside the CGWW. Red and blue light had the most ratios and accounted for 23.9% and 27.1% of the spectrum distribution inside the CGWW, respectively, and both of them had an increase compared with outside. And red and blue light had transmittances of 32.4% and 37.5%, respectively, which were far higher than both UV-A and far-red light. Due to the selective permeability of the water-walls for solar spectrum, obviously, high temperature inside the CGWW could be controlled while adequate photosynthetically active radiation could be ensured. In addition, the CGWW showed some of regularity in distributions and daily variation of water-walls temperature and air temperature inside the CGWW. In summary, the CGWW which can obtain an ideal cooling effect, suitable humidity and illumination conditions via its own structure, has been proved to be a feasible, low carbon and energy saving greenhouse type, and will provide a reference and technical supports for the development of closed greenhouses.