Abstract: The Bistatic SAR (BiSAR) acquires the radar cross section of target area from different directions, which is helpful for SAR image classification and target recognition. The spotlight mode has higher resolution and thus has advantages in target recognition. This paper introduces an advanced hyperbolic approximation method to fit accurately the cubic term of range history. Besides, the wavenumber domain signal is derived and the image formation process is given. Then, the residual phase term induced by range variance property is shown, and the compensation step is accommodated in the modified algorithm, which enhance the imaging performance at the edges of scene. The simulation results verify the accuracy and validity of the proposed method.