Abstract: The corrosion behaviors of steel bars in 40 Si2Mn alloy in concrete under different external stress conditions were studied, and the influencing factors and mechanism of corrosion and cracking of steel bars were analyzed. The results show that after the reinforced concrete specimens were immersed in 3.5% NaCl solution for 250 days, and when the applied stresses were 0%, 20% and 40%, the specimens didn't fracture and the number of surface cracks was less than those of the concrete specimens without steel bars; when the applied stresses were 60% and 80%, obvious cracks could be seen on the surfaces of the specimens and the cracks were wide; the impedance value decreased gradually with the increasing of the stress in the same corrosion time;with the increasing of the stress, the corrosion current densities of the samples and the corrosion area of the surface of the steel increased; when the stress was 80%, larger micro cracks appeared on the steel surface, and the steel was severely corroded and the bearing capacity declined.