Abstract: Based on the geometric relationship between the axial displacement, the transverse displacement and the shear strain in an elastic body, the bending normal stress formula of the sandwich beam is derived, and the allowable bending moment of the sandwich beam can be determined by the bending normal stress formula. Here, the shear effect is duly considered unlike the allowable bending moment calculation in the textbooks of materials mechanics. It is shown that the accuracy of the formula is high and the shear effect has a certain influence on the allowable bending moment. Thes results can be used in the teaching of the sandwich beams in the course of materials mechanics.