Based on the principle of heat transfer theory and finite element method (FEM), a model for power cable thermal field and ampacity computation has been developed by adopting the soil region division method. The soil region around the cable is divided according to the difference of the soil thermal resistance. Based on the structural parameters of XLPE cable and the physical parameter of the laying area, different layers temperature distribution of XLPE cable is analyzed through the model, and the dichotomy is employed to calculate the cable ampacity. Comparing the calculated result by this method with the calculation by the standard, shows that compared to IEC60287 standard, the model has high-accuracy and is more convenient to the external environment. Through simulation example, the corresponding relationship between the influencing factors and the cable ampacity is obtained, which provides important theoretical basis for the optimization of the cable laying.