The range walk correction (RWC) in time domain can resolve range walk problem in high squint SAR imaging. However, the limit of focus depth exists. By analyzing the residual phase error after wave number decoupling and Doppler rate error caused by RWC, a refined azimuth non-linear chirp scaling (ANCS) algorithm is proposed. Phase error caused by the third order range migration is corrected and a nonlinear CS function is applied to equalize the targets Doppler rates before azimuth compression. Simulation results show that the refined ANCS algorithm has good performance.