The flexural stiffness of magnesium alloy is one of the most important assessment indexes for its engineering application. Three-point support method was adopted to investigate the flexural stiffness and Young's modulus of both 2.16mm and 3.66 mm AZ31 and AZ31B magnesium alloy sheet. It indicates that the flexural stiffness and Young's modulus of AZ31 and AZ31B magnesium alloy are almost the same. The flexural stiffness range is from 28.4GPa to 31.1GPa, while the range of Young's modulus is between 41.74 and 45.1GPa. The same experiments were carried out on aluminum alloy and steel samples with the same surface density as that of 3.66mm magnesium alloy sheet, which showed that the magnesium alloy had the largest bending resistance factor. The specific stiffness of AZ31 magnesium alloy was a little higher than that of AZ31B magnesium alloy and 93.2% of that of steel while 20.6% higher than that of aluminum alloy. Therefore, magnesium alloy is a very suitable material for light-weight design. The results have provided a solid experimental support for its wide application and lightweight design.