In stepped-frequency high resolution ISAR imaging radar, target motions produce the range-Doppler coupling which has a serious effect on the ISAR image. Consequently, a novel motion-parameters estimation method based on the image contrast optimization is proposed. Firstly, the effect of radical velocity and acceleration on Doppler profile contrast function was analyzed. Secondly, after constructing phase compensation term, the radical acceleration was estimated by the Doppler profile contrast function. After the radical acceleration compensation, the radical velocity was estimated by the range profile contrast function. Finally, after motion compensation and range-Doppler algorithm, the high resolution ISAR image was obtained. This method has some advantages of high accuracy and fast computation speed. The experimental results demonstrate the effectiveness of the proposed method. 