A three dimensional numerical simulation is conducted to predict the thermal environment of a high temperature supersonic combustion tunnel. The numerical results show that the high temperature combustion gas expands at the contoured nozzle exit, causing a local high temperature domain between the nozzle exit and the catch cone entrance. The supersonic high temperature combustion gas converts into subsonic flow through a shock train structure in the diffuser duct, and the flow separation occurs near the exit of supersonic diffuser under the extremely adverse pressure gradient. The wall heat flux fluctuates with the position of shock waves, and its distribution along the diffuser seems a "saddle" shape, which means the heat flux is lower in the second throat section and higher at the entry and exit of the supersonic diffuser. To achieve the repeated use of high temperature supersonic combustion tunnel suffering from high heat flux numerically predicted, the thermal protection is designed with adoption of boiling heat transfer method and verified by experiment. The result shows that the method of boiling heat transfer can protect the diffuser from the heat flux of 3MW/m² by the subcooled water with a velocity of 2m/s. The application of boiling heat transfer can make the design much easier, which can achieve a better heat transfer effect at a lower speed and pressure loss of the subcooled water.