The chatter dynamics of a two?degree?of?freedom model in the high?speed cutting process was analyzed. A set of four?dimensional piecewise equations with the segment of cutting force were constructed for the model at first.Afterwards, the influence of the dynamic component of the cutting force on the cutting chatter was studied, and the condition of the chatter was derived by analyzing the associated characteristic transcendental equation. It reveals that the Hopf′s bifurcation occurred at a positive steady state as bifurcating parameters pass a sequence of critical values.And the results of the analytical calculations are verified by full numerical simulations.