Abstract: In this paper,a simple discrete vegetation model is used to estimate the change of soil moisture. First,we use an assumption to replace the total volume of the backscattering from vegetation cover.Second,we ignore the double bounce com ponent of the model of its little contribution to the whole backscattering of VV polarization;thirdly,the last term of the model, the backscattering from bare soil,is described as a function of the Fresnel reflectivity and surface roughness functions. After comparing the two repeatedly passed measurements,we get a new algorithm to estimate the relative change of soil moisture. The results show that the algorithm proposed in this paper can be used directly to estimate soil moisture change and magnitudes of the vegetated surface using multi-temporal L-band VV polarization AirSAR data.