In order to study the yaw instability of FPSO in regular waves, nonlinear fully coupled 6DOF motion equations of moored floating body are established and solved numerically. The accuracy of equation is verified by comparing equilibrium yaw angles calculated by numerical simulation with model test. The influence of yaw instability on motion responses is studied. The influences of initial yaw angle and wave steepness on yaw instability and motion responses are also studied. The results show that the FPSO may lose the weathervane effect at particular wavelength to ship length ratio, and this increases the wave loads acting on the hull which arouse greater responses in roll and heave. The wave steepness doesn’t affect the final equilibrium yaw angle. However, it affects the time required to reach equilibrium and the response amplitude. The initial yaw angle affects the final equilibrium position and motion response of the FPSO.