Abstract: According to the predigestion model of mechanics of the semi-rigid nodes of beam-column end-plate in steel frame,and based on experimental study on 3 different extended end-plate connections,the thesis presented the non-linear mathematics model of the moment-rotation relationship curve with the initial stiffness and ultimate load-bearing capacity for its primary parameter.The thesis gived the ultimate flexural resistance using subassembly method and initial stiffness,and compared with the compution result and experimentation result using three-folden-line model,,validated the calculating method of the initial stiffness and ultimate load-bearing capacity.