In this paper, the cooling effect of seawater spraying on the offshore launching platform used by the CZ-8X rocket at different takeoff heights is studied. The rocket plume model using the realizable k- ε turbulence model is established, which is based on the three-dimensional Navier-Stokes equations. The injected seawater for cooling the deck surface is described by the Eulerian dispersed phase (EDP) model. The numerical results are compared with the experimental data in the literature to verify the validity and accuracy of the numerical model. The coupled flow field of the CZ-8X rocket plume and seawater at different takeoff heights and different spray velocities are simulated. The results show that the thermal environment on the deck surface of the platform is the worst when the takeoff height reaches 30 m. The seawater spraying velocity shall be moderate before the takeoff height reaches 30 m. A low spraying velocity will lead to poor cooling effect on deck surface, and a high spraying velocity could lead to increased diffusion of plume flow in non-spraying direction. After the takeoff height rises to 40 m, a low water spraying velocity makes the injection position of seawater far away from the jet core area, and the seawater evaporation rate is low, which is conducive to the accumulation of seawater layer on the deck surface. The cooling effect of thermal barrier layer is very obvious. The research shows that when the rocket is launched at sea, the launch platform can be cooled by pumping and spraying seawater, and the water spraying rate can be adjusted appropriately according to the takeoff height to achieve the best cooling effect.