As China’s power consumption continues to expand rapidly, high-voltage cables are assuming a more critical role in transmission and distribution networks. However, the existing thermal state evaluation methods of high-voltage cables have limitations: owing to its neglect of the complex heat transfer phenomena in the air gap layer, the conventional IEC 60287-based thermal circuit method yields results of insufficient accuracy, thereby failing to meet the rigorous requirements of contemporary power system models. Therefore, this paper presents a FEM-based method to accurately calculate the equivalent thermal resistance of the air gap layer. According to the actual topological structure of high-voltage cable and the principle of heat transfer in different materials, a refined thermal circuit model of high-voltage cable is established. The heat transfer mode of corrugated aluminum sheath, air gap layer and buffer layer structure is analyzed emphatically. The equivalent thermal resistance of air gap layer is solved by FEM numerical simulation method, which replaces the calculation results of coaxial cylinder formula in IEC standard, and further gives a universal fitting formula of equivalent thermal resistance of air gap layer. Based on the result, the maximum error of the ampacity between the optimized thermal circuit model and the FEM results is only 3.5% under different cable types, different arrangement environments and arrangement modes, and the temperature distribution error is controlled within 3 °C, which demonstrates the model’s high accuracy and robustness across a range of conditions.