This paper investigated the damage behavior of AZ31 magnesium alloy plate by numerical simulation method by finite element software DEFORM, which is based on variable gauge rolling. The rolling temperature of magnesium plate is 350℃,the rolling speed is 20r/min and the lateral pre-deformation by vertical roll are 37.5%,75% and 112.5%, respectively. Then the specimens that have been pre-deformation are rolled by flat roll with multi cycles of small reduction or single cycle with a large deformation. The results showed that the variable gauge rolling process can significantly reduce edge damage of rolled magnesium plate. When vertical roll reduction is 75%, the variable gauge rolling process reach the best, and that the maximum edge damage factor is 0.35. Variable gauge rolling process seems to more suitable for large reduction rolling. The maximum damage factor is only 0.126 when vertical reduction is 75% and flat reduction is 49% were combined to produce magnesium plate. The value of vertical roll reduction was related to the initial thickness of magnesium plate.