The physical explanation of shear lag is given in this paper. The shear lag effects can be attributed to the abrupt change of shear stress at web-flange junction. The abrupt change at the junction of shear stresses changes the magnitude of the flange bending stresses, which attenuates gradually with increasing distances from the web. Meanwhile, the abrupt change of shear stresses from zero to the flange shear stress also influences the bending stress state at the centerline of the flange. The method to determine the shear lag effects has been given, which is based on the Euler-Bernoulli elementary theory of bending, and the calculated results are in good agreement with the experimental results. Considering the transversal distribution of the shear stress at the junction, the flange real bending stress could be calculated on the section. The change of the shear stresses decreases the magnitude of the bending stresses in tension, but increases the magnitude of the bending stresses in compression. The shear lag effects in continuous box girder are different from that in the simply supported box girder. The reaction force of the continuous support would generate an abrupt change of the shear forces between the two sides of the support, and the real bending stresses beside the support would vary significantly due to shear lag effects. The ratio of real bending stress to standard bending stress is not the proper index to reflect the shear lag effects, but the real bending stresses could represent the shear lag effects and should be paid more attention to in design.