By shearing Q235 steel, aluminum, and AZ31 magnesium alloy at room temperature, the shear area of Q235 steel and aluminum is found to be relatively flat whereas that of AZ31 magnesium alloy exhibits many defects, such as potholes and cracks. The influence of temperature and strain rate on the critical fracture strain of AZ31 magnesium alloy was obtained using isothermal compression experiment. Results show that high temperature and larger strain lead to large and small critical fracture strains. Therefore, based on the isothermal compression experiment and the effects of temperature and strain rate on the critical fracture strain of AZ31 magnesium alloy, the magnesium alloy plate is heated to 100, 200, 300, and 400 °C, and shearing was conducted after 30 min of heat preservation. Based on the cross-sectional shape and the degree of damage, the optimum shear temperature ranges from 160 °C to 260 °C. At this temperature, the sheared magnesium alloy plate not only obtains an improved cross-sectional shape but also has a small shear corner area. Simultaneously, the shearing basic process model of Q235 steel plate is also obtained based on the industrial test. Furthermore, the shearing basic process model of AZ31 magnesium alloy was acquired based on the elongation ratio of magnesium alloy and Q235 steel under the same process conditions.