The hydrodynamic coefficients and heaving damping of a truss spar platform were studied by the computational fluid dynamics (CFD) method with considering the semiclosed moon pool. A partial model was established to investigate the added mass and viscous damping of platform heaving; a full model and a simplified model were established to study the nonlinear damping of platform heaving. The influences of opening ratios of moon pool on the added mass and viscous damping of platform heaving were analyzed. It is found that the natural period and the heaving added mass of the platform increase if the semiclosed moon pool is considered. Different opening ratios of the moon pool will lead to different heaving damping of the platform and dynamic pressure acting on the guide plate of the moon pool. In practice, an optimal opening ratio of the moon pool can be designed. The numerical results were verified by comparing with the results of model experiments.