Shot blasting on a tube steel of TP304H can greatly improve the oxidation resistance at 650℃-770℃ in water vapor, by the presence of an appropriate blasting intensity. SEM and EPMA analysis indicated that the structure of the oxide scale turned from multi-layer to monolayer after blasting and Cr2O3 in predominant. The untreated sample characterized in multi-layer oxide was composed of Fe in the outer layer and Fe and Cr underneath. For blasted one, the increase of diffusion rate of Cr and the decrease of effective diffusion energy of scale led to a uniform Cr-rich oxide layer, which was dense and protective and the oxidation resistance was increased.