Range cell migration (RCM) correction (RCMC) is the key process in the imaging formation of bistatic synthetic aperture radar (SAR) (BiSAR). Due to the existence of double square root in the bistatic range equation, the RCMC in the general BiSAR configuration is more complicated than the traditional monostatic method. In this letter, the RCM of BiSAR system is analyzed in detail, and a novel data transformation is derived. From the derivation, we find that the original keystone transform (KT) is only a special case of our proposed transformation, so that we called it the generalized KT (GKT). Compared with the original KT, this generalized method can correct more complex RCM nicely. After the GKT processing, the RCM is eliminated closely, and the focused image with full range and Doppler resolution can be obtained using the fast Fourier transform. Finally, a hybrid spaceborne/airborne BiSAR numerical simulation is performed to validate the proposed processing procedure.