Abstract: Abstract: The wall plays an important role in the study of the greenhouse. Heat storage layer and thickness of the wall are 2 key factors for studying wall thermal environment and optimizing the structure. As widely used wall in solar greenhouse, soil wall is typical and representative, and it is significant to study heat storage layer and thickness of soil wall. To study the varying rule of the heat storage layer and to find the optimum thickness of the wall, an experiment was performed in Yanghe County, Yinchuan, Ningxia, China in 2011. The fifth generation greenhouse with soil wall from Shouguang City, Shandong Province was used. The CR10X-2M and CR3000 data collectors from America were adopted, and 18 T-types thermocouple temperature probes were also used to test inner wall temperature at the positions of 0, 10, 20, 30, 40, 50, 60, 80, 100, 150, 200, 230, 250, 280 and 330 cm thickness (along horizontal direction of wall body). The temperature variation was analyzed in the horizontal direction. Indoor temperature and wall temperature were used to determine the thickness of heat storage layer. Besides, a method for calculating wall thickness was pointed out, which was based on wall temperature spreading speed. The results showed with the increase of wall depth, the variation of bilateral temperature-wave of the wall became less obvious. However, the temperatures were gradually decreased from wall surface to exterior surface. Temperature-wave amplitude approached zero at 50-230 cm depth of the wall surface, which was in a stable state. The thickness of heat storage layer was got by using the lowest temperature of inner greenhouse and inner wall. The temperature changed obviously in 0-50 cm and 280-330 cm depth of the wall, which showed wave layer, stable layer and insulation layer existing in the north wall of solar greenhouse, and the thickness of heat storage layer was various. In January, the amplitude of temperature within a month at wave layer ranged from 10.01 to 0.34 ℃ with an average temperature amplitude of 3.30 ℃, and the average temperature was 11.70 ℃. The amplitude of temperature within a month at stable layer ranged from 0.17 to 0.10 ℃ with an average temperature amplitude of 0.08 ℃, and the average temperature was 4.76 ℃. The amplitude of temperature within a month at insulating layer ranged from 0.29 to 5.18 ℃ with an average value of 2.73 ℃, and the average temperature was -9.16 ℃. The thickness of the heat storage layer in experimental greenhouses was 55-200 cm and the average thickness was 109.2 cm. With the rising of the temperature outside, the thickness of heat storage layer was gradually decreased. The average thickness of heat storage layer was 144 cm in January while it was 100.9 cm in March. The thickest was 200 cm in January, while the thinnest was 55 cm in March. The most suitable thickness of the soil wall was 1.5 m in Yinchuan area, which was calculated by using the temperature wave propagation velocity. In practical production, the soil wall should be 250 cm thick at the bottom, 100 cm thick at the top and 150 cm thick at the middle.