Based on the experimental investigations of the capillary absorption of water and chloride diffusion of steam cured concrete, the microstructure difference between surface layer and inner concrete was analyzed. The surface layer degradation effect and its mechanism of steam cured concrete were also discussed. The test results show that, compared with the standard cured concrete, both the capillary absorption of water and chloride diffusion rate of steam cured concrete are much higher at the surface layer（about 10mm deep from the surface）. It can be found that the microstructure of surface layer is significantly damaged during steam curing, and the degradation effect mainly concentrates on the range of 10mm deep from the surface. The main reasons for surface layer degradation effect of steam cured concrete are the larger temperature gradient and the expansive deformation in concrete surface layer. The incorporation of fly ash and slag into concrete can alleviate the degradation of surface layer during steam curing, and effectively increase the resistance to ingress of water and chloride ion.