To ensure the stability of the system under the combined wave-current, it is necessary to fully consider the dynamic response of the riser-wellhead system in practical engineering design. For the complex environmental conditions of a typical sea area in the East China Sea, a finite element simulation model of the drilling riser-underwater wellhead system is established by considering the mechanical properties between the drilling riser and the wellhead structure. The study focuses on the structural stress and deformation characteristics and the change of the wellhead force under different environmental loads and top tensions, and the influence of the wellhead installation deflection angle on the structural stability is explored. The results show that the sensitivity range of the drilling riser to the top tension varies under different environmental loading conditions, and the structural stability can be effectively improved by increasing the top tension within a certain range; the underwater wellhead force is related to the installation method of the wellhead device, and the equivalent bending moment generated by the top tension due to the installation deflection angle plays a dominant role.