A two?degree?of?freedom oscillator system with plastic impact is considered. The existences of non?smooth bifurcations of the system are analyzed， and the periodic motion patterns and existence regions are identified in the （ω，δ）?parameter plane.The bifurcation characteristics between adjacent periodic motions and dynamics in the hysteresis and subharmonic inclusions regions which lie between the （ω，δ）?parameter domains of （1，0，0） and （1，1，0） motions are analyzed. The bifurcation behaviors such as codimension?1 crossing?sliding， switching?sliding and multi?sliding bifurcations and codimension?2 sliding bifurcation in the impact oscillator are revealed. In the plastic impact case， non?sticking and sticking single?impact periodic motions transit into each other through crossing?sliding bifurcation. There exists a group of narrow hysteresis domains along the boundary of the subharmonic inclusions region， and the connection point of adjacent domains of hysteresis is a double?grazing bifurcation and flip?fold bifurcation point. Switching?sliding and multi?sliding bifurcations of the impact oscillator are manifested as rising bifurcations， but there is a difference in the location of rises occurring in the sticking phase. In two?parameter plane， the intersection point of two types of sliding bifurcation curves is a codimension?2 sliding bifurcation point.