Abstract: According to Symplectic Elasticity，the wing plate of simply-supported T beam is assumed in the state of plane stress. Saint-Venant analytical solutions of wing plate under uniform or linear load are deduced respectively. The closed polynomial analytical solution of shear lag factors in terms of beam length, beam width and Poisson ratio is obtained. Taking a simply-supported T beam with wide wing as an example, the shear lag factors are calculated using both the formulas introduced here and FEM. A comparison between the formulas results and FEM results verifies the correctness of the deduced formulas. It is convenient and rapid to calculate shear lag factors of simply-supported T beam bridges using the dedeuced formulas.