Abstract: An experimental study of an excited parametric vibration is reported,the resonance condition is that external frequency is twice as large as inherent frequency, and the direction of driving force is vertical to movement direction. The amplitude-frequency characteristic of the resonance and the critical state are discussed. When both intensity of vertical excitation and frequency width number are small, the law of vibration is sensitive to frequency width number . While frequency width number is reduced, amplitude number will oscillate with time. That is ＂beat＂ phenomenon. Both period number and peak-valley difference number increase with the decrease of frequency width number. After ＂beat＂ disappears, the amplitude number continues increasing exponentially with time number. When frequency width number equals to zero, the amplitude number increases sharply with the period number, the vibra- tion is unstable and then resonance occurs. Based on the pendulum model ,the numerical solutions of dynamic equa- tion are in well agreement with the experimental results. The experimental study for solving problems such as pre- vention of natural disasters associated with resonance has potential applications.

Key words: vibration, double frequency, numerical solution