The physical explanations both of positive or negative shear lag are given in this paper. The shear lag effects occur attributing to the abrupt change of shear stress at junction upper the web. The abrupt change of shear stress or called shear stress jump changes the stress magnitude of the flange bending stress, which attenuates gradually at increasing distances from the web. Meanwhile, the abrupt change of shear stress from zero to the flange shear stress also influences the bending stress state at the centerline of the slab. The method to determine the shear lag has given, which is based on the Euler-Bernoulli elementary theory of bending. Considering the transversal distribution of the shear stress at the junction upper the web, the real flange bending stress could be calculated all on the section.