For nonlinear Duffing system, a derived synchronization system is built and the method is proved. By the synchronization, the faint difference in stimulating signal of Duffing system can be detected. In simulation, when there occurs chaos in the original system, the responses of the original and derived system will exhibit synchronization for identical input signal and non-synchronization for different input signal. When the system is in quasi-periodic motion, the synchronization is still sensitive to the faint variation in the input signal. The simulation results prove the effectiveness of the method.