The frequency of extreme weather events in China has been increasing due to global warming. The variety of soil types makes for varying levels of water retention capacity across Southwest China. However, spatially detailed information about soil water retention capacity is often difficult to obtain on a large scale. It has been hypothesized that agricultural drought is directly related to evapotranspiration fluxes and soil water retention capacity. This paper presents the relative water supply capacity index (RWSI), which is based on the integrated standardized precipitation evapotranspiration index (SPEI) and vegetation condition index (VCI) to explore the potential of using the temporal moderate resolution imaging spectroradiometer archive and meteorological data collected from stations in Southwest China to estimate the soil water retention capacity. The results showed that the spatial pattern of RWSI correlated well with that of the soil clay content in Southwest China, which suggests that RWSI could reflect the variations in relative soil water retention capacity in the research region. The RWSI was significantly correlated with relative soil moisture at depths of 0 to 10 cm and 10 to 20 cm. These data showed that RWSI could be used to examine and distinguish the relative differences in soil water retention capacity with an integrated implication of VCI and SPEI in Southwest China.