Using ZS-1 type real-sea erosion-corrosion test equipment which is designed by ourselves, the 72-hour continuous erosion-corrosion experiments of 3C steels have been done at Zhoushan marine corrosion test station. Electrochemical impedance spectroscopy (EIS) measurement has been used to detect the corrosion of samples. The results show that the corrosion rate of 3C steel is decreasing with the erosion-corrosion time when the flow velocity is 1 m/s, 2 m/s and 3 m/s. In the velocity range of 1 m/s~6 m/s, the corrosion potential of 3C steel increases with the velocity increasing, electrochemical impedance spectroscopy has single capacitance impedance and shows activation controlled characteristic. With increasing flow velocity, the shear stress on the metal electrode surface and the electrochemical reaction impedance decreased. It shows a linear relation between polarization resistance (Rp) and flow velocity.