The corrosion behavior of Corten - A weathering steel was investigated by immersion - dry - wet accelerated corrosion test in 0.3 % magnesium chloride solution. By using SEM and EDAX technology, the corrosion images as well as the contents of the chloride ion and magnesium under different cycles were provided. It ap- peared linear relationship between the weight loss and corrosion test cycles. In early stage the main crystal product is/β - FeOOH, in late period it is γ- FeOOH with few ε - Fe2O3. The result of the electrochemical tests showed that the cathodic reaction was accelerated due to the reduction of the rust; the large amount of the rust indicated that structure of the rust was loose and the rust did not have protection ability.