The surface treatment of AZ91D magnesium alloy was carried out in KF+KOH treatment solution using the AC micro-arc oxidation treatment technique. The relationship between coating thickness and treatment parameters was established, the structure of the coating was determined, the growth mechanism of coating was discussed and the treatment parameters for quick forming of coating was optimized. The results show that under the conditions of 979 g/L for concentration of KF, 349 g/L for concentration of KOH, 81 V for voltage and 32 ℃ for the temperature of treatment solution, a 30 μm-thick compact coating which is mainly made up of MgF2 and MgO can be formed on AZ91D magnesium alloy surface after 88 s. This forming speed of the coating is 7 times quicker than that of other treatment techniques at least