Inverse synthetic aperture radar (ISAR) increases the target rotation angle to improve the azimuth resolution. Since the echo data points are fan-shaped in wave number domain, the effective interpolation region is broadened in azimuth direction and compressed in range direction while the target rotation angle increases in the process of Polar Format Algorithm (PFA). This gives rise to the reduction of range wave number bandwidth, which causes the range resolution loss. In this paper, an imaging algorithm based on the varying-parameter method is proposed to eliminate the change of range wave number caused by the rotation angle. This method makes the echo data points wedge distributed in the wave number domain by adjusting radar platform parameters in azimuth direction at each azimuth sampling point. Thus, the compression of range wave number width is effectively reduced. Finally, the simulations are performed to verify that the varying-parameter method can restrain the deterioration of range resolution to improve the quality of image.