The slab of the composite girder is usually very wide in composite cable-stayed bridges, and the main girder has an obvious shear lag. There is an axial force in the main girder due to cable forces, which changes the normal stress distribution of the composite girder and affects the shear lag. To investigate the shear lag in the twin I-shaped composite girder (TICG) of cable-stayed bridges, analytical solutions of TICGs under bending moment and axial force were derived by introducing the additional deflection into the longitudinal displacement function. A shear lag coefficient calculation method of the TICG based on additional deflection was proposed. Experiments with three load cases were conducted to simulate the main girder in cable-stayed bridges. And the stress, deflection, and shear lag coefficient obtained from the theoretical method considering additional deflection (TMAD) were verified by the experimental and finite element results. A generalized verification of a composite girder from existing references was made, indicating that the proposed method could provide more accurate results for the shear lag effect.