Abstract: The internal structure of soil affects the heat transfer process and the ampacity of buried cables. Therefore, the laying conditions are taken into consideration, and an electro-thermal-fluid fields model is established for studying the internal structure's effects of soil on the operation of buried cable. Firstly, a thermal conductivity model of soil was established based on the heat transfer theory in porous medium. Secondly, the variations of temperature distribution and ampacity of ±500 kV high-voltage DC cable were studied under different soil porosity, water-air ratio and permeability. Finally, the results were compared with those from the model of IEC standard to analyze the limitations of IEC model. The results show that the increase of soil porosity and the decrease of water-air ratio enhance the ratio of air-liquid within the pores, leading to a decrease in the soil thermal conductivity. This, in turn, results in an elevated cable temperature. In extremely case of dry soils, the maximum elevated temperature in steady-state cable is about 25 ℃. The increase in soil permeability changes the heat transfer mode from single heat conduction to heat conduction and convection composite processes, thus the steady state temperature changes in cable can vary by up to approximate 20 ℃. As a result, the IEC standard for cable ampacity calculations can be specifically applied to conditions with low soil permeability, low porosity, and high water-air ratios. The research findings can provide a theoretical basis for the selection of cable laying locations in complex environments, maximizing the ampacity of buried cables, and enhancing their operational reliability.