Abstract: Under hogging moment, the tensile stress of the reinforcement bar in the roadway slab varies with its distance from the girder web, which is termed as ‘shear-lag’. Taking into account the tension stress varying through the thickness, a method for defining the effective flange width for the double composite section is proposed. 29 double steel-concrete composite beams are analyzed using nonlinear finite element method, and the effective widths of the reinforcement bars in the hogging moment zone near the middle support point are determined at different loading stages until collapse. Some dominant parameters are analyzed to illuminate the influence of effective width, and then a simplified calculating method at the plastic stage is proposed.