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.