The kinematics numerical simulation model of Lingshui 17-2 semi-submersible platform is established, and the free decay motion with different degrees of freedom is simulated. In order to ensure the reliability of the model, the scale ratio model test is used to check the simulation model, and the simulation results of the free decay motion response under given initial conditions are compared with that of the model test, which are basically consistent. Based on the free decay curve, the damping coefficient of the platform is calculated and the damping force (moment) time history curve is numerically analyzed. The results prove that the offshore motion evaluation of Lingshui 17-2 platform could be carried out based on the simulation model.