SUnder the influence of shear lag and local stability, the longitudinal stress distribution of flanges of steel plate beam bridges is not uniform. Combined with engineering examples, the average stress is too small if calculated by the elementary beam theory, and there is a potential safety risks. The distribution along the transverse direction of bending stress can be obtained accurately by using the finite element method, but the modeling process is complex and the workload is large. According to the current code, the method of effective distribution width is adopted to simplify the calculation, and the result is conservative, which can be used as a reference for engineering design.