Abstract: Preparing phase change composites by mixing expanded graphite powders and paraffin can improve the heat transfer performance of the energy storage materials effectively. In order to investigate the thermal conductivity mechanism of expanded graphite/paraffin phase change composites, the calculation method of the effective thermal conductivity coefficients of three scales solid after the mixture of expanded graphite powders and paraffin was put forward. Then, by numerically simulation calculation, the effective thermal conductivity coefficients of expanded graphite/paraffin phase change composites containing expanded graphite thermal conductive particles with different volume fractions and different thermal conductivity coefficients were obtained. The results show that expanded graphite can improve the heat conduction performance of paraffin greatly, and when the volume fraction of expanded graphite is 10%, the effective thermal conductivity coefficient of expanded graphite/paraffin phase change composite is as high as 9 times of the pure paraffin. In addition, improving the mixing degree of graphite sheets and paraffin in bottom scale and reducing the volume fraction of the bottom scale graphite both can improve the effective thermal conductivity coefficient of expanded graphite/paraffin phase change composites effectively. The conclusions obtained lay a foundation for the investigation of the mechanisms of expanded graphite powders improving the thermal conductivity coefficients of phase change composites.