Abstract: The as-cast AZ31 magnesium alloy was high strain-rate (3.6-10.4 s^-1) rolled at preheating temperatures of 300-450℃ and rolling linear speed of 828 mm s^-1 with single pass reductions of 10%-80%, and the microstructure and mechanism of microcrack evolution were investigated. The results show that twinning was the main deformation mechanism at early deformation stage, and the twin number sharply increased; with increasing rolling reduction, recrystallization occurred and the increase of twin number was gradually stable. Twin density in magnesium alloy microstructure decreased with increasing preheating temperature. Twinning induced recystallization and bulging recrystallization were the main recrystallization of magnesium alloy. The competition among twinning, recrystallization and microcrack existed. In fine grain area, the formation of crack was attributing to the initiation, growth and coalescence of holes. Twinning induced microcracks, while the formation of many twins could inhibit the crack propagation.