The corrosion behavior has been studied on the three different coupled samples dipped in 5%NaCl solution. The coupled samples are Q235 steel with AM50 alloys, one without any coating, the other with cathodic E and cathodic electrophoretic coating on magnesium alloy. The surface morphologies of magnesium alloys with three different surface conditions have been observed in detail after dipped. The corrosion rates have been measured and the microstructure has been examined after ending the corrosion dip. The phenomena and law of galvanic corrosion have been explained by corrosion model. The results show that the cathodic electrophoretic coating could postpone the galvanic corrosion of magnesium alloy with steel effective and pre-silane treatment with much better protection. The affection area of galvanic corrosion did not increase with the dipping time. The corrosion developed more quickly along the coupled area than along the surface extending.