Based on a semi-submersible platform operated at a water depth of  1.5 km, five truncated mooring systems were designed for the model test, which have similar horizontal restoring force characteristics, vertical force-horizontal displacement characteristics and top tension-horizontal displacement of single line as the full depth system, but different restoring moment characteristics. The timedomain coupling analysis method was applied to obtain and compare the roll and pitch motions using these different truncated mooring systems. The simulation results were compared with model test data. It is concluded that moment characteristics similarity has a significant impact on the roll and pitch motions of a semi-submersible platform. And the impact increases as the horizontal offset of the platform becomes larger. Therefore, keeping similar moment characteristics with the full depth system is essential while designing the truncated mooring system.