A new method for identifying stable cutting speed in regenerative chatter is studied by time- varying cutting width. The model of the regenerative chatter system is established in CNC lathe. In this model, all dynamic parameters had been obtained by experimental methods. The cutting system stability is simulated and the relation curves between the limited cutting width and spindle speed are established in the experiment. Simulation and experimental results show that it can identify stable speeds range effectively and such stable speeds range make the cutting process in stable state. The method is applicable and credible to suppress chatter. The testing works were performed in a CNC lathe.