Abstract: A one-third scale specimen of composite steel frame-reinforced concrete infill wall structures with semi-rigid connections was tested under cyclic load. The failure mechanisms were analyzed and a simplified calculation method for lateral ultimate strength was proposed based on plastic theory, in which the interaction effects between steel frames and infill walls, semi-rigid connections, studs and diagonal struts forces were considered. The finite element models using ANSYS software were setup for non-linear analysis. Through a comparison of experiment results, plastic theory results and finite element results, it was found that the calculated values by plastic-theory were stable, and they underestimated the lateral ultimate strength by an amout of 7%-8%, implying the simplified calculation method is feasible. Finally the failure mechanisms were revealed.