Abstract: In the drilling process in deep water, the tenser system of the riser provides the constant top tension to accommodate the stress and deformation of the riser, so it is the key factor to ensure the stability of subsea wellhead system.The mechanical analysis model and the deflection differential equation for subsea wellhead system were established after The influences of ocean current, platform offset, seabed soil and top tension were studied comprehensively.Then, the bending moment and transversal displacement characteristics of riser system under different top tension ratios (tension ratio: 0.9, 1.0, 1.1, 1.2, 1.5) were analyzed based on the theoretical model.Finally, the influence of top tension on the stability of subsea wellhead was analyzed by using the method of mechanical equivalent transformation.It is indicated that when subsea wellhead is installed in place, the top tension of axial load is the only controllable factor influencing the stability of subsea wellhead if the platform offset is constant.The stress and deformation of riser and subsea wellhead can be improved further by increasing the top tension of the riser.Based on the calculation method of minimum top tension, it is beneficial for the stability of subsea wellhead system in the field of deepwater drilling to set the tension ratio at 1.0-1.5.