Abstract The dynamical properties of the forced Duffing oscillator disturbed by small sig-nal near the critical point of the non-equilibrium phase change are expounded. The phasecondition of the phase change happening is deduced. It is concluded that the phase trans-form is sensitive to the small signal near the critical point. The frequency difference condi-tion of the intermittent chaos happening is deduced. The small frequency difference nearthe critical point is the reason of the intermittent chaos appe...