This paper studied the equivalent mechanics model of sloshing motion of water inside the moonpool of a deep sea Spar platform, and the model parameters were calculated. The dynamic equations of water inside the moonpool were derived based on the potential flow theory. The equivalent pendulum models of the sloshing motions of water inside the moonpool were established. ANSYS software was used to set up model and mesh, and the Matlab software was used to carry out the numerical calculation. The Galerkin method was used to solve the natural frequencies, modal functions and discrete solutions of the potential function of the sloshing motions, and the model parameters of the equivalent pendulum were calculated. The validity of the calculating method was verified by comparing the numerical simulation results with the analytical results. The parameter library of the equivalent mechanics model was established considering the different water heights of the moonpool. This lays the foundation for studying the coupling motions between the platform and water inside the moonpool.