Abstract: In order to analyze the potential impacts of transverse cracks on structural safety of Composite box girder, Engineered Cementitious Composite（ECC） numerical model was built using ABAQUS program. This paper aims to reveal the change of shear lag of steel-ECC composite box girder and analyze the influencing factors of shear lag effects. The results show that with the increase of load, the shear lag effect of steel-ECC composite box girder gradually increases, but when the strain of ECC flange plate reaches the limit state, the shear lag effect begins to stabilize. In addition, the shear lag effect of steel-ECC composite box girder can be effectively reduced by appropriately reducing the width and thickness of flange plate or increasing the height of box girder web. Meanwhile, compared with two-point symmetric loads, concentrated loads lead to greater shear lag effect. These research results can provide theoretical support for designing safer and more reliable steel-ECC composite box girder.