This paper focus on the mitigation of porpoising inststability of high-speed planing vessels using periodic force. Firstly, a heave-pitch model of planing vessel is constructed and the hydrodynamic coefficients are analysed. After that, the basic theory of using periodic force to mitigate porpoising is presented. The open-loop simulations of mitigation of porpoising using periodic force are shown to verify its rationality. To make the moment imposing the periodic force more accurate, close-loop simulations are performed to feed back the pitch information which can be detected. This close-loop system makes the mitigation of porpoising more effective.