In this paper, the authors deal with the property of the dynamic response of a semisubmersible platform with a large list angle in beam sea condition. In the previous experimental tests, it was observed that a significant subharmonic motion appeared in the response for certain wave frequencies. In order to make clear the mechanism of the unstable motions, we have carried out the numerical simulations of the response in regular waves. It becomes clear that the subharmonic motion is mainly caused by the nonlinear coupling moment due to the difference of hydrodynamic forces acting on the both-side lowerhulls. Furthermore, the calculated results agree qualitatively with experimental ones.