Under the planesection assumption and the premise that no relative slip exists, a calculation theory of shear stress in the variable depth and width box sections with corrugated steel webs based on elastic beam elements is given. The calculation theory proposes that shear, moment and axial force will all produce shear stress, and the latter two can produce shear stress only in the case of variable cross section. For further analysis, a finite element model is established, and a comparison among the traditional calculation theory, the calculation theory and the results of the finite element model is conducted. The calculation theory is proven to be reliable.