Abstract: In order to calculate rationally the shear stresses in prestressed concrete box beams with variable cross section and reflect objectively their distribution, general formulas for shear stresses of arbitrary non-uniform beams are derived. The practical simplified formulas for shear stresses of box beams with variable cross-section are presented with the variations of beam depth, thickness of web and base plate. The derived formulas are applied to investigate the stress distributions of cantilever beams with variable rectangular and box cross-section. It is shown that, large shear stress occurs in the base plate of a box beam with variable cross-section, and the maximum shear stress of a non-uniform beam doesn’t exist at the location of gravity axis but in the region below the gravity axis or at the bottom fiber. In the design of box beam with variable cross-section, thin base plate shouldn’t be used, and the arrangement of reinforcements in the base plate should be strengthened.