Thermal washing is a common method of wax removal in oil fields. The law of phase change heat transfer of wax during the thermal washing process is revealed to be of great significance for improving the melting rate of wax. The lattice Boltzmann method is used to numerically simulate the process of phase change heat transfer in pipe-lines with different angles (90°, 45°, 0°, –45°, and –90°) of wax layers based on the enthalpy-porous medium model in the present work. The boundary condition between wax and hot water is considered as convective heat transfer boundary. The effect of the Prandtl number on the law of phase change heat transfer of the wax at various angles is investigated. The simulation results indicate a non-linear decrease in the complete melting time of the wax layer from –90° to 90°. The heat transfer capacity is enhanced with the decrease of Prandtl number, which effectively shortens the melting time of wax. The complete melting time of the wax layer is reduced by 23.78% when the Prandtl number decreases from 79.4-59.4. The speed of convective heat transfer is increased with the decrease of Prandtl number, which means that the solid zone of wax is accelerated into the loose and porous mushy zone, and the efficiency of wax removal is improved. The research results can provide a reference for improving the efficiency of thermal washing.