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Based on the principle of vegetation water supply index (VWSI), relative normalized difference vegetation index (RNDVI) and relative land surface temperature (RLST) are proposed, with which a modified vegetation water supply index (MVWSI) is constructed, i.e., M V W S I = R N D V I R L S T 2 Compared with VWSI, MVWSI eliminates the difference caused by the spatial and temporal distribution of VWSI. MVWSI was applied to monitor drought over Sichuan and Chongqing, China in 2006, suggesting that MVWSI can demonstrate the whole process of drought from its emergence and development to its weakening and disappearance. The relative precipitation index (RPI) is used to examine the effectiveness of MVWSI. The examination result shows that MVWSI and RPI have high consistency in spatial distribution and in-site time series analysis. Compared with vegetation condition index (VCI), temperature condition index (TCI), vegetation temperature condition index (VTCI) and temperature vegetation drought index (TVDI), MVWSI is simpler in calculation and more stable hence more easily realized in application.