The relationship among stress, cavities and twinning in the room-temperature bending process of hot-rolled AZ31 Mg alloy sheet was researched. It can be seen that the mount of twins which was tightly affected by stress value increases with the distance to press point. Otherwise, the cavities and twinning which seem totally independent, in fact affect with each other. Some cavities were hindered by twins and can not further grow up. Then, the relationship between slip and twinning was also roughly discussed. The dislocation lines in twins can be thought to release the stress focus and were beneficial for deformation.