Soil moisture is a direct source of water for terrestrial plants. Measurement of soil moisture can help to characterize plant growth and portray regional soil moisture information in a timely manner, which is of great significance for water and soil regulation. This study uses the Tropic of Cancer in Yunnan Province as a research area, and based on MODIS product data, the apparent thermal inertia (ATI), vegetation supply water index (VSWI) and surface water capacity index (SWCI) are calculated. The measured data are used in regression analysis modeling to retrieve the soil moisture in the study area. The results show that the three indices of ATI, SWCI and VSWI have good correlation with soil moisture, among which VSWI has the best fitting effect, and ATI and SWCI can retrieve soil moisture more effectively. The soil moisture contents are mostly concentrated in the range of 0.2 to 0.4 m³/m³, among which the soil moisture of Jianshui County, Mengzi City, Wenshan County, Yanshan County, Gejiu City and Shiping County are the smallest, and the soil moisture in the western portion of Gengma County and Cangyuan County and the central portion of Jinggu County are the largest. As the fractional vegetation cover increases, the soil moisture content also gradually increases, and the mean value is greater than 0.2 m³/m³.