To solve the problems of the simulation in time domain and the consistent movement between waves and ships,based on frequency-spectrum analysis method,a novel method for calculating the ship oscillation movement is proposed,in which directional spectrum is used instead of using ocean wave spectrum.Mathematic expressions and the relationship between wave spectrum and directional spectrum are introduced and the long-crested wave spectrum and ITTC directional spectrum suitable for ship oscillation movement analysis are selected.The ocean wave is decomposed using Gerstner model,and then the simulation model of ship movement in the waves is built.The relationship between encounter wave surface and ship movement is described by response amplitude operator method.With the linear superposition assumption,the results of ship oscillation movements in frequency domain and time domain are obtained.