Abstract: The research is based on a composite large-span double-tower single plane cable stayed bridge with the span of (54 m+108 m+340 m+108 m+54 m) in Zhejiang Ningbo Zhoushan port, the main girder of which faces the problem of a complex main girder with a large aspect ratio and remarkable shear lag effect. To study the shear lag effect of the cable-stayed bridge and the influence of the width-variant girder on the shear lag effect, we studied the main mechanical behavior and stress distribution of the main girder by combining the mixed finite element method and field measurement, and obtained the stress and shear lag coefficient distribution of the top and bottom slab in the finished state. The results show that the shear lag effect in the top and bottom slabs of the single-box triple-room cable stayed bridge is significant. With the increase of the width of the main girder in the widening section, the shear lag coefficient of the top slab decreases continuously. For the steel floor, the change of the number of U-ribs of the floor caused by the change of the width of the main beam leads to an uneven stress distribution of the bottom slab. Therefore, the arrangement of the longitudinal ribs of the floor can be optimized according to the distribution curve of the shear lag coefficient. The results are significant and provide useful reference for the design and construction of this kind of bridges.