Abstract: The constraint equations at the interface between beam elements and shell elements are derived and the corresponding mixed finite element method is formulated based on the plane-section assumption. The internal forces of the whole beam structures can be computed by beam element method and then conveniently applied to the members for the analysis of local stresses. A steel box member with longitudinal ribbed stiffeners is taken as an example, in which the mixed finite element method is adopted to analyze the local stresses and the local stability of panels. Conclusions are drawn on the necessary length of the member part described in shell element and its reasonable distribution in the mixed finite element model.