Magnesium alloy is one of the most useful structural materials in the future. But the research report about surface strengthening of magnesium alloy is little in home and abroad. Ultrasonic impact strengthening treatment was carried out on AZ91D magnesium alloy, the change characteristics of surface microstructure was studied using SEM. The scratch hardness and wear-resistance were tested for both ultrasonic impact treatment and un-treated specimen using HRA-150 type Rockwell hardness tester and M-2000 model wear tester. The experimental results indicate that after ultrasonic impact, the grain boundary on the surfaces of AZ91D is obviously refined. When the ultrasonic impact time are 20 min, 30 min and 50 min, the scratch hardness on the surface of AZ91D increases 7.2%, 16.96% and 28.90%, respectively. For ultrasonic impact time of 50 min, the wear-resistance of treated specimen could be increased to 39.63% and 37.56% respectively for the wear time for 2 and 5 min. With increasing the ultrasonic impact time, the wear scar on the surface is become narrow. It is an effective method to increase the scratch hardness and wear-resistance of AZ91D magnesium alloy by using ultrasonic impact strengthening treatment.