Abstract: Double-level bridge decks offer significant economic advantages and simplify traffic management. Over time, double-level cantilever slab box girders have been increasingly utilized in steel bridge construction. To investigate the effect on shear lag effect of steel box girders with double-level cantilever slabs, the shear-stiffness warping displacement function is chosen and adjusted based on the double-level cantilever box girders' shear flow distribution. A differential equation describing the shear-stiffness effect of double-level cantilever box girders is created using the energy variational principle, and an analytical solution is derived based on boundary conditions. Using a simply supported steel box girder as a case study, the correctness of the analytical theory was verified using ANSYS finite element analysis. The results indicate that the calculations based on the analytical theory and the finite element method are in good agreement. Moreover, the shear lag effect of a conventional box girder can be analyzed using the degeneration approach proposed in this study, which accounts for only the upper cantilever slab. The presence of the lower deck cantilever slab reduces the stress on the bottom plate of the box girder and somewhat alleviates the shear lag effect compared to a traditional box girder with only the upper deck cantilever slab. As the height-to-span ratio increases, the shear lag coefficient of the top and bottom plates changes little, while the normal stress in the top and bottom plates decreases significantly. As the thickness of the lower cantilever slab increases, the normal stress in the mid-span section of the bottom slab decreases significantly, while the shear hysteresis coefficient gradually increases, indicating a more pronounced shear hysteresis effect. As the width of the lower cantilever slab increases, the maximum positive stress in the bottom slab decreases, but its shear lag coefficient increases, making the shear lag effect more pronounced.