Adopting process of tagnite, effects of three current controlling, modes, namely constant current, unipolar current and bipolar current on properties of anodic coatings of AZ91HP magnesium alloys were studied, coatings formed by the three methods were all compact, the number of pores per unit by bipolar current was the least among them , and there was no difference between those by constant current and unipolar current. The uniformity of pore size for the coating obtained by bipolar current was the best and that by constant current was the worst. According to EDS analysis, these coatings contained Si, O, Mg and F. Salt spray test showed that corrosion resistance of the coating by bipolar current was the best and the coating by constant current the worst.