The cast AZ61 magnesium alloy and the extruded alloy were chosen to study the corrosion morphology and mechanism by means of SEM, XRD and electrochemical testing, under the NaCl atmosphere at 120 oC and 1.8 MPa. The results show that due to the multi-factor coupling effects of NaCl atmosphere, temperature, pressure and microstructure, the corrosion rate of the extruded AZ61 magnesium alloy is larger than that of the cast AZ61 alloy, because of the re-distribution of β phases and smaller grain size. But both of the corrosion products are mainly Mg(OH)2. The environmental factors play an important role in the corrosion behavior of magnesium alloys. Higher temperature and sodium chloride atmosphere work together to accelerate the corrosion rate of Mg alloys