The effect of asymmetric rolling on the grain refinement of AZ31B magnesium alloy was investigated. The changes of macromorphology and grain size at different temperatures and reduction rates were analyzed and compared with that of symmetrical rolled alloy. The experimental results indicated that the overall grain size of the asymmetrical rolled alloy was finer than that of symmetrical rolled alloy. The grains was the finest and uniformity when the asymmetric rolling was at the temperature of 350 ℃ and the reduction rate of 60%, and the average grain sizes of the upper surface, the center layer and the lower surface were 2.35, 2.84 and 2.22 μm, respectively. The dynamic recrystallization of the structure occurs in the range of 300350 ℃. With the increase of rolling temperature, the grain boundary migrated and diffused sufficiently, and the grain grew, which led to the deterioration of the comprehensive properties of magnesium alloys. The tensile strength and elongation of asymmetrically rolled sheets were better than that of symmetrically rolled sheets. When the reduction rate was 30% at the rolling temperature of 400 ℃, the comprehensive mechanical properties were excellent, and the tensile strength was 365.36 MPa and the elongation at break was 34.9%.