Temperature distribution and ampacity are two important parameters to reflect the operating state of submarine cables. According to the ±160 kV HVDC submarine cable of Nan’ao project, a three-dimensional (3D) simulation model of the cable is set up and a simplification method of cable transient model is proposed by using COMSOL Multiphysics finite element analysis software. On the basis of the constant thickness, multiple thin layers and adjacent medium are used as three equivalent layers. The thermal conductivity of the equivalent layer is computed by harmonic mean method,and the specific heat capacity and density of the equivalent layer are calculated by IEC 60287 method, which solve the problem that the meshs in the thin layer of the cable are too dense. Through the simulation model, the temperature distribution of cables under different ampacity is compared before and after simplification, which verifies the correctness and effectiveness of the simplification method.