Abstract: This paper analyzes the shear lag effect of box girder bridge based on the stiffened plate theory, and establishes the bar simulation method to study the shear lag effect of single box double cell box girder. A stiffener area formula and the shear lag differential equation are derived for the single box double cell box girder, based on a comparison of examples of the organic glass single box double cell box girder model for the shear lag effect in the numerical solution of plate and shell, and in the analysis of the experimental solutions and analytical solutions. The reliability and the accuracy of the bar simulation method in the study of shear lag effect of single box double cell box girder are verified.