More than 95% Solar Greenhouse walls were built with earth in Northwest China, the thickness of the earth walls changed with areas. In order to find out the optimal earth wall thicknesses of Solar Greenhouse in the Northwest China, the study chose some earth wall of solar greenhouses used widely in the Northwest China from Tacheng City in Xinjiang Uigur Autonomous Region, Yangling Region in Shanxi Province, Baiyin City in Gansu Province and Yinchuan City in Ningxia Hui Autonomous Region. The environmental indexes of the regions were compared and analyzed, and the heat release at night and heat transfer characteristics were researched The results showed that the optimal thickness was 1.0 m in Yangling Region, 1.3 m in Baiyin City, 1.5 m in Yinchuan City and 1.4 m in Tacheng City, mainly based on the cost of construction and land utilization rate considered comprehensive. These thicknesses can meet the requirements of the local insulation property. It is not significant to increase the thermal insulation effect with again increasing the thickness when reached the optimization.