Subsea well mouth system is an essential equipment for deep⁃sea oil and gas drilling and production. Conventional casing hanger and sealing assembly running tools cannot meet the actual needs. The mechanical model of running tool for casing hanger and sealing assembly was established, which can directly describe the force on the running tool. According to the mechanical model and analysis results, the structure of running tool could be well designed. The contact and strength analysis on casing hanger carrying and driving unit of running tool were conducted using ANSYS finite element analysis software. The analysis results show that the maximum stress values of key components, such as locking block, retaining ring and mandrel of casing hanger, are less than the yield strength of material, which deformation is small enough to be ignored. The structure of the running tool meets the design requirements, and the material used selection also meets the operation standards. This work can provide design experience for the running tool of casing hanger and sealing assembly.