The offshore launch platform considering the thermal launch of a rocket at sea has been studied. The Realizable k-ε turbulence model and the three-dimensional potential flow theory are used to study the time-domain motion response of the platform under the impact load of the rocket wake and the combined loads of the wind, wave and current. The heave, roll, pitch and yaw of the launch platform during offshore launching under two sea states are compared and analyzed. The analysis and calculation show that one sea state is more beneficial to the two-way safety of the rocket and the launch platform than the other, and the launch platform should avoid offshore thermal launching in beam waves. In addition, a simple calculation method of the wake impact under offshore launch condition is proposed to obtain the pressure time history of the gas wake impact for single Laval tube, providing theoretical and engineering foundation for the subsequent study of the structure and motion performance of the launch platform.