To improve the inherent resolution of synthetic aperture radar (SAR), the relationship between the spetrums of bistatic SAR (BiSAR) was analyzed and a novel range resolution auto-combination method was proposed in this paper. Based on the proposed method, the whole procedure was divided into two steps: First, the raw echo of BiSAR was well focused by an modified nonlinear chirp scaling algorithm, where no interpolation was required. Secondly, an auto-combination technique based on image quality was introduced to combine sub-images into an improved resolution image. Simulation results show that this technique is effective and robust.