Abstract: Thermal hydraulic process without and with helium cooling first wall of 3D model light water cooled subcritical energy blanket driven by ITER is numerically studied with 3D distributions of fuel power density and power flattening via turbulence simulation with RNG k-ε model and fluid/solid heat transfer coupling method. The present results show that cooling the first wall with helium can significantly reduce the maximum temperatures of the solid material. The highest temperature distributions of fuel have the same variation trends as the fuel power density. Power flattening is beneficial to equally distribute the mass flow in the cooling channels. A thermal hydraulic design with enough thermal margin is obtained in this paper.