Crop Water Stress Index (CWSI) that was firstly suggested by Jackson and Idso to evaluate water status was expressed as a function of (T_c-T_a).Since remote sensed thermal radiation is a mixture information of soil and canopy, CWSI is not applicable at the pixel scale.On the assumption that land surface temperature (T_S) is a linear function of canopy temperature (T_C), soil surface temperature (T_soil) and vegetation cover fraction (f_v) and that there is no coupling between soil and canopy, Water Deficit Index (WDI) that was extended from CWSI was used to evaluate regional drought.In the paper, air temperature (T_a) spat ial patterns in the first ten days of April and second ten days of May in 2000 were firstly figured out with NDVI-T_s (where NDVI is Normalized Difference Vegetation Index) space with Terra MODIS land surface temperature and vegetation index product.Then Water Deficit Index (WDI) was calculated based on (T_s-T_a) and NDVI.The comparison between WDI and topsoil moisture show that WDI can reach a satisfactory estimation of field water deficit for both full-covered and partially vegetated sites and have overcome the shortage of CWSI that can only work for the site of fully covered by vegetation.