The stability of riser and wellhead system plays a crucial role in the safe and highly efficient production of offshore oil and gas, in order to study the mechanical behavior of riser and wellhead system under actual working conditions,in this paper, a theoretical coupled mechanical model of riser and wellhead system is established by considering the factors such as wave load and soil resistance, and analyzed the influence of the tension and the displacement of the platform on the mechanical behavior of the riser and the combined pipe in formation.The analysis results show that the offset of platform has a little effect on the lateral displacement and bending moment of the riser and the combination pipe,increasing thetension can reduce the lateral displacement and bending moment of riser, and it also can reduce the lateral displacement and bending moment of combined pipe which can enhance the stability of the wellhead system.