Abstract: To evaluate more exact system reliability of steel frame,taking the actual semi-rigid joint as a rotational spring element,formulae of the reduced stiffness matrix and the equivalent nodal force of beam element with semi-rigid joints are deduced.The method of branch boundary is adopted.The strength reliability of the semi-rigid steel frame system is studied and compared with that of fully-rigid and pined connections.The example indicates that there are real differences in the main failure path,its failure probability and system reliability between the semi-rigid connections and the fully-rigid or pinned connections and that the effects of joint stiffness on the strength reliability of steel frame are important.