Abstract: Nowadays, equivalent linearization method is widely used, but the computed result always has great deviation while dealing with common offshore engineering sites with deep soft surface layers. In order to show the extent of the deviation, we choose a typical borehole and use equivalent linearization method to analyze the site responses under the excitation of minor, moderate and major ground motions. In this procedure, borehole models with or without soft surface layers are considered. As we known, soil has large variation range of nonlinear behaviour under severe earthquakes and equivalent linearization method can not fit well, so a direct time-domain integral method is introduced and compared with equivalent linearization method. Based on the above analysis, some solutions to the determination of design ground motion for offshore engineering sites with deep soft surface layers are proposed and discussed.