To study the rotation mechanisms of the steel column, end-plate, and end-plate stiffener of extended end-plate joints, cyclic loading tests were carried out on two extended end-plate joints, one of which contained a composite slab. The deformations of these three components during test loading were described using digital images, and their bearing capacities and rotation angles in the elastic stage, yield stage, and strengthening stage were deduced by the component method. The results showed that the failure modes of the extended end-plate joints were yielding of the end-plate, bolt fracture, and end-plate stiffener buckling, and the composite joint experienced local crushing of the composite slab. The composite slab improved the initial rotational stiffness and bearing capacity of the composite joint. The joint rotation angle of the extended end-plate connection was mainly composed of three parts: shear deformation of the panel zone, relative deformation between the end-plate and the column flange, and yield deformation of the end-plate stiffener. The composite slab had a constraining effect on the deformation of these three parts. In addition, by defining the moment–rotation curves of three components, a simplified three-rotation spring model was established to simulate the joint-loading process. The results showed that the model could simulate the test joint-loading process accurately, and is conducive to promoting seismic performance analyses of prefabricated steel frames.