In this paper, analysis of the broadening effect of the first-order sea clutter in the HFSWR (High Frequeny Surface Waves Radar) radar system of moving shipborne platform is given. The platform is moving in six degrees of freedom (DOF) and the model of movement has been built. The technique of STAP (Space-Time Adaptive Processing) is proposed to compensate for the platform movement, and the motions like roll, pitch and yaw of the ship have been mainly discussed. The analysis of the performance of this algorithm is put forward, and the simulation results show that the space-time processing can get a better clutter suppression effect, which can improve the performance of the target signal detection.