The nonlinear motion of the Spar platform hull is studied by numerical simulation and model experiment in this paper. The nonlinear differential equation for a coupled heave-pitch of a platform hull is established in a regular wave. The bifurcation pictures of response amplitude with wave frequency variation, Poincare’ maps and time histories are calculated to study the nonlinear dynamical behaviors of the platform hull. The parameters domain for unstable pitch motion is calculated numerically. It is found that the platform motion is sensitive to wave frequency. With the changing of wave frequency the platform exhibits harmonic motion, twice super-harmonic motion, 1/2 sub-harmonic motion, quasi-periodic motion and chaotic motion. The nonlinear 1/2 sub-harmonic motion and the parameter domain for unstable pitch motion are qualitatively verified by the model experiment.