This paper proposed a new focusing approach for the bistatic SAR systems with a GNSS (Global Navigation Satellite System) satellite as the transmitter. Because of the long integration time the inherited curvature of the transmitter's trajectory was adopted and the spatial variance of the equivalent velocity was derived under the principal of equivalent slant range model in the system signal model. The proposed focusing approach consists of two-step processing. Firstly, the general Doppler phase from constant velocity of the echo data is focused by modified Stolt mapping. Secondly, the residual nonlinear Doppler phase due to velocity variation is compensated by means of block-processing-based adaptive phase compensation to achieve large scene focusing. The simulation and experimental results validate the proposed algorithm in GNSS-based BiSAR systems.