Abstract: Driven by ocean wave, the swing of a ship possesses characters of multi-cycle and randomness. In this paper, imaging method based on pendulum module is proposed for dealing with these problems. This approach develops images from linear frequency modulation components that are included in the phase history in every range bin. To compensate the three and higher order phase errors, rank two phase estimator is presented based on the available theory of rank one phase estimator. A robust auto-focus algorithm is also presented based on the Randon-Wigner algorithm and the idea of CLEAN. The effectiveness of the algorithm is verified by processing of real data.