The dynamic analysis of a Tension Leg Platform(TLP) in random wave is investigated by considering the set-down of a floating body.The nonlinear restoring stiffness is derived with the set-down motion of a floating body and the coupled motion of the tension leg and platform and the differential equations of the motion are established.The study focuses on the influence of the set-down motion on the nonlinear response of the platform.By considering different significant wave heights and currents,motion responses of the platform are calculated and compared.The analysis shows that the set-down motion significantly increases the heave motion with low frequency and the equilibrium position of the heave motion with the set-down motion is much lower than that without set-down motion.The results in this paper indicate that the set-down motion has a major impact on the safety of the platform inproduction operation,and it is also a threat to the strength of tension legs and risers. The dynamic analysis of a Tension Leg Platform (TLP) in random wave is investigated by considering the set-down of a floating body. The nonlinear restoring stiffness is derived with the set-down motion of a floating body and the coupled motion of the tension leg and platform and the differential equations of the motion are established. the study focuses on the influence of the set-down motion on the nonlinear response of the platform.By considering different significant wave heights and currents, motion responses of the platform are calculated and compared. the analysis shows that the set-down motion significantly increases the heave motion with low frequency and the equilibrium position of the heave motion with the set-down motion is much less than that without set-down motion. The results in this paper that the set-down motion has a major impact on the safety of the platform inproduction operation, and it is also a threat to the strength of tension legs and risers.