A　semi-analytical　method　is　provided　to　predict　the　behavior　of　beam-to-column　semi-rigid　connections　of　steel　structures.　A　calculation　formula　of　beam-to-column　connection　rigidity　is　derived　firstly　based　on　the　theoretical　analysis　of　semi-rigid　connections.　With　the　derived　theoretical　formula　of　connection　rigidity　and　a　series　of　experimentally　measured　values　of　loads　and　displacements　at　the　end　of　beam,　an　effective　semi-analytical　method　for　the　prediction　of　semi-rigid　connection　rigidity　of　steel　structures　is　presented.　The　proposed　method　can　eliminate　the　difficulties　and　the　accuracy　errors　in　directly　measuring　the　joint　rotation　angles　in　the　traditional　experimental　methods,　and　can　predict　more　actual　performance　of　semi-rigid　connections　than　the　finite　element　method.　An　example　is　given　to　show　the　simplicity　and　the　accuracy　of　the　semi-analytical　method　presented　for　obtaining　the　M-θ　curves　of　beam-to-column　semi-rigid　connections　of　steel　structures.