The changes of microstructure, microhardness and surface roughness of AZ31B magnesium alloy after ultrasonic surface rolling process (USRP) were investigated. And their effecton the internal microstructure of AZ31B magnesium alloy in the process of tensile test. A gradient deformation layer was generated on the surface of AZ31B magnesium alloy by USRP. The microstructure, microhardness and surface roughness of the samples were analyzed by X-ray diffractometer, EBSD, microhardness tester and surface roughness tester. Tensile tests were carried out on the untreated and USRPed samples to analyze the tensile properties and microstructure deformation. After 9 passes of USRP, a deformation layer about 200 μm was formed on the surface of the sample. The microstructure of the deformed layer is gradient distribution:ultrafine grain layer, fine grain transition layer and base material. With the increase of the distance from surface of the sample, the microhardness of the sample surface is between 84HV10g and 52HV10g. The surface roughness of USRP specimen reaches Ra 0.140 μm. Compared with the untreated sample, the microstructure changes of the USRPed sample are less. Compared with untreated samples, the yield strength and microhardness of the samples treated by ultrasonic surface rolling are increased by 28% and 40%. The surface roughness was reduced by 96.4%. And, the gradient structure formed by USRP on the surface of the magnesium alloy sample can reduce the deformation of the central microstructure during the tensile test.