The effect of slip of the steel-concrete interface and shear deformation of the steel webs were considered to accurately analyze the shear lag effect of the double-cell composite box girder. The different shear lag warping displacement functions for the top and bottom plates are defined. Based on energy method, the control differential equation and the closed-form solution for the double-cell composite box girder are derived. As an example, the shear lag effect of a single box double-cell composite box girder is analyzed by using proposed method. The results show that the deflection of the composite box girder when considering the slip and the shear deformation is obviously larger than that obtained by the elementary beam solution, and which will decrease with the increase of interface slip stiffness. The slip value is approximately proportional to load heavy. The effect of shear lag on bottom steels lab of the box girder is more significant than that on top concrete slab.