Micro-FTIR mapping technology was used to monitor the amount and distribution of Mg(OH)2 on the anodic coating of magnesium alloy which was immersed in the 7.3 Wt% Na2SO4 solution for different time. In the solution, part of the MgO on the surface of the Mg alloy could gradually transform into Mg(OH)2 which could be detached from the Mg alloy surface and dissolved into the solution. With immersion time of 2 h in 7.3 Wt% Na2SO4 solution 2h, FTIR mapping results showed that FTIR absorption signal of Mg(OH)2 was strongest and Mg(OH)2 was most on the surface of the anodic coating. After 4 hours, the content of Mg(OH)2 began to decrease, and the Mg alloy was etched gradually. The FTIR mapping results of another component Al2O3 with immersion time were almost similar to those of Mg(OH)2. The impedance of the oxide film was also analyzed using electrochemical impedance spectroscopy. It showed that the impedance changed with the immersion time and conformed to the corrosion law of the oxide coating. This research has a good guidance and application value for characterization of the anodic coating on magnesium alloy.