Abstract: The effect of H₂O(g) content on the surface oxidation layer and decarburization behavior of 55SiCr spring steel was studied by thermos gravimetric (TG) test. The results show that the decarburization degree of 55SiCr spring steel in mixed atmosphere was higher than that in air, the thickness of ferrite decarburization layer increases with the increase of H₂O(g) content in mixed atmosphere, and the peak value of ferrite decarburization layer thickness appearred at 850 ℃. Compared with dry air, the presence of H₂O (g) in the mixed atmosphere changed the composition and structure of the oxide layer, so that the specific gravity of the oxidation product increased toward FeO. The structure of FeO is loose, which will increase the degree of decarbonization.