In order to avoid the approximation of spheroid surface as flat plane in calculation of sampling volume,the differential theory is applied to derive the expression of effective sampling volume for bistatic weather radar system.It is proved that the ratio of the bistatic resolution volume to the monostatic resolution volume,or V_b/V_m,numerically equals the derivative of the polar axis length (R_t) to the semi-major axis length (a) in the bistatic radar system.Though the result from numerical calculation for sampling volume is the same as that from references,approximations performed to obtain the expression has been eased and the derivation process is theoretically strict and easy to understand.Some relationship expressions between the geometric elements for bistatic polarization radar system,such as the ratio V_b/V_m,the radius R_t and co-plane azimuth Φ for the transmitter,the radius Rr and co-plane azimuth Φ_r for the receiver,and the bistatic angle β,are obtained.The features of effective sampling volume variation with the related geometric elements are analyzed.The results provide a theoretical base for bistatic polarization weather radar antenna control,data processing,sidelobe contamination elimination and detecting accuracy improvement.