In natural seawater without and with 0.01mol/L H2O2, the corrosion behavior of high-strength low-alloy (HSLA) steel was investigated by electrochemical polarization, Raman spectroscopy, X-Ray Diffraction(XRD)and rotating ring-disk electrode (RDE) experiment. And the corrosion mechanism of steel in each solution was discussed. The results showed that the free corrosion potential and limited diffusion current density of steel increased obviously through adding H2O2, meanwhile the hydrogen-evolution potential also shifted negatively. The compositions of rust layer in two solutions were almost identical, and the oxygen reduction process of the steel in two systems were both controlled by the two-electronic reaction. In conclusion, adding H2O2 to natural seawater could efficiently accelerate the corrosion of steel as the similar corrosion process as in natural seawater.