With the rapid development of the national economy, cables have become the main channel for urban power transmission. In the context of the construction of modern power systems, the requirements for the digitization and intelligence of electrical equipment are improved, and digital modeling technology is urgently needed for high-voltage cables to enhance ampacity and safety and stability. The thermal state is a key limiting factor for the ampacity of cables. The existing researches usually construct the thermal model of cables based on the thermal circuit method. However, the traditional steady-state thermal circuit model based on the IEC 60287 standard has low calculation accuracy and is difficult to meet the actual engineering requirements. In this paper, based on the actual topology of the cable, especially the buffer layer-air gap layer-corrugated aluminum sheath structure, a refined steady-state thermal circuit model for high-voltage cables incorporating an air gap layer has been developed, and the effectiveness analysis is carried out. The refined thermal circuit model of YJLW03-Z 127/220 kV high-voltage cable is established, and the radial temperature distribution and actual ampacity of this type of cable are calculated. The results demonstrate that the refined steady-state thermal circuit model established in this paper effectively resolves the ampacity calculation errors caused by the omission of air gap layer structures in traditional thermal circuit models, and greatly improve the calculation accuracy of the thermal circuit method. The difference between the ampacity calculation error and the actual value is not more than 1.4%, and the temperature distribution error of each layer structure is not more than 2 °C.