In order to improve the thermal conductivity of thermoplastic composites and provide basis for the development and application of thermoplastic composites, a method for determining the equivalent thermal conductivity of thermoplastic composites was studied. Thermoplastic composites were prepared by using polyimide molding powder, colloidal graphite and carbon fibers as raw materials. An experimental platform for measuring thermal conductivity in complex environment was built. Thermal conductivity of a single material was calculated theoretically by steady-state method. The thermal conductivity of thermoplastic composites filled with graphite and carbon fibers is very different. A general method to determine the equivalent thermal conductivity of thermoplastic composites is needed. On the basis of the equivalent structure, the equivalent thermal conductivity of thermoplastic composites was determined according to the constant physical properties, no internal heat source and steady state heat conduction. The equivalent thermal conductivity was calculated from bottom to top. Carbon fiber filled thermoplastic composites and graphite filled thermoplastic composites were measured by an experimental platform. The results show that the measured results are basically distributed around the measured data. The equivalent thermal conductivity of thermoplastic composites is measured by adding experimental errors satisfying normal distribution. The results show that the proposed method can give accurate results of the equivalent thermal conductivity of thermoplastic composites under measurement errors. It can be seen that the results of the proposed method are accurate.