Abstract: The chaotic motion of nonlinear coupled heave-pitch of the classic Spar platform hull is studied by the Lyapunov exponent in this paper. The nonlinear differential equation of the coupled heave-pitch of the platform hull is established in the regular wave. Taking the classic Spar as an example, the maximum Lyapunov exponent spectrum and the bifurcation diagrams are calculated and the parameter domain for unstable pitch motion is calculated. The results show that the platform motion is sensitive to wave frequency. With changing of wave frequency the platform exhibits 1/2 sub-harmonic motion and quasi-periodic motion. When the wave frequency approaches the natural frequency of heave, the platform exhibits chaotic motion.