Liquid metal thermal grease is a promising thermal conductive material for decreasing contact thermal resistance. However, there exist great differences between liquid metal based thermal interface materials and traditional polymer based thermal interface materials, such as the interface thermal resistance between the base fluid and fillers, and the ratio of the thermal conductivity of the base fluid and fillers. The thermal conductivity models for traditional polymer based thermal interface materials are not applicable to liquid metal thermal interface materials. In this paper, an approximate MG model (AMG) is derived to describe the thermal conductivity of liquid metal thermal interface materials. The error between the predicted thermal conductivity and the simulation result is within 6%; In addition, the effects of filler type, diameter, volume percentage and interface thermal resistance on thermal conductivity were simulated. It was found that when the filler diameter was within 100 nm-5 µm and the interfacial thermal resistance is less than 10^-8 (m²·K)/W, micro/nanoparticles doping can significantly improve the thermal conductivity of liquid metal thermal interface material. This study will provide theoretical guidance for promoting the thermal conductivity of liquid metal thermal interface materials.