Abstract: Based on the strength theory of materials, some formulas for shear stress calculation in prestressed variable depth beams are introduced and demonstrated in this paper. The physical concept of each term in the formulas is clarified. An analysis, a comparision and evaluation for all formulas are given. At the same time, shear stress distribution is studied. Finally the theory and formula of bend-shear stress calculation in prestressed thin wall box beams with variable depth are introduced and it is indicated that other formula is inadequate.