The environmental loads such as wind, wave, flow onto drilling rig and mooring system of the HaiYangShiYou 981 are analyzed according to the actual working condition in South China Sea. The Haiyang Shiyou 981 drilling rig is chosen as the research object for mooring positioning system. The 3D model of the platform is established first by using the AQWA software to simulate and analyze the motion response of the drilling device under the action of wave load. As the deep water drilling rig is fixed in the sea with twelve anchors which are symmetrical distribution, the influence of top pretension angle on dynamic response of mooring system and motion response of drilling rig under different environmental load is analyzed by changing the inclination angle of the pretension at the top of the drilling rig.