The AZ31 magnesium alloy tube is used to electromagnetic forming experiment under the five kinds of input voltages, and cause of crack formation is analyzed. The analysis result shows that: adiabatic shear behavior and deformation localization are crack mechanism; electric difference, mechanical heterogeneity, uneven loading, and incomplete symmetry coil geometry are causes of crack formation. The ways of avoiding crack and increasing deformation degree are to improve comprehensive properties of the tube and to adjust deformation conditions during electromagnetic forming.