Through analyzing the influence of imaging of 3D rocking ship targets under heav y scale sea, as well as based on the concept of variance, an average Doppler spr ead definition is proposed to estimate the optimal processing time and duration for ISAR. In this algorithm, the ship natural pitching period is estimated th rough the information of the range direction, by which the micro Doppler eff ect is eliminated and a reliable basis for smoothing Doppler spread curve is pro vided. The algorithm can work automatically instead of artificial intervention. The simulation and measured data processing results show that ideal ISAR images can be obtained by using the data segments selected by the proposed algorithm.