For a chaotic system with unknown amplitude and frequency in external exciting force, the unknown parameters were expanded to be the new state vector and a new drive system was constructed. By a parametric adaptive control, a response system with the same structure as the drive system was constructed. Based on the stability theory in the cascade system, controllers and the parametric adaptive law were designed by two steps, which make the driveresponse systems achieve complete synchronization. Then the unknown amplitude and frequency of the drive system can be identified. The forced DuffingVan der Pol oscillator and loudspeaker system were taken as examples to illustrate the effectiveness of the proposed method.