Abstract: Single-box twin-cell box girders differ significantly from single-cell box girders due to their complex structure of two cavities.Even under symmetric loading, the twin-cell box girder will still produce positive symmetric distortion effect.In order to accurately analyze the influence of this effect on the transverse internal force, this paper expands on the basis of the frame analysis method of single-cell box girder, and proposes a new analysis method of transverse internal force of the frame for the case of symmetric loading on the top plate about the middle web, and constructs the corresponding equivalent calculation model and basic formulae with full consideration of the structural characteristics of the twin-cell box girder.The reliability and accuracy of the proposed calculation method are verified by actual engineering cases and compared with the finite element method.In this paper, the influence of two parameters, the web inclination angle and the middle web thickness, on the transverse internal force are further discussed.The analysis results show that the calculation method proposed in this paper is consistent with the distribution pattern of the results obtained by the finite element method and has high accuracy.The transverse internal forces caused by the positive symmetric distortion effect should not be neglected, and its influence cannot be accurately reflected by relying on the traditional calculation method of elementary mechanics framework.Under positive symmetric loading, compared to inclined-web twin-cell box girder with equal width of the top plate, the transverse internal force of the rectangular twin-cell box girder reaches the peak at the intersection of the top plate and the side webs, whereas it shows a smaller value at the other points of the calculations.The variation of the middle web thickness has little effect on the final transverse internal force distribution.