Three dimensional turbulent fluid flow and heat transfer in the cooling channels of high enthalpy and pressure air heater were numerically investigated. The liquid water was employed as the coolant, and the materials of combustor and nozzle wall were steel and copper respectively, whose thermo physical properties vary with temperature. The hot-fire tests of the facility were performed to validate the simulation results. The influences of gaseous radiation, the channel configuration and cooling routing on the wall heat transfer were discussed and analyzed. The result shows that the heat transfer of combustor wall is affected obviously by the gaseous radiation which has little effect on the nozzle wall. The result will lead to significant error if the gas radiation is neglected. The heat transfer rate of wall is largest if the number and the width of channel was optimized. The flow mode of coolant has a great effect on the coolant side wall temperature but a little impact on the hot-gas side wall temperature.