In view of the high rate of false alarm problems on the distributed optical fiber sensing technique applied in enclosure, a sensor is proposed based on double Mach-Zehnder interferometer and a method is needed urgently to identify intrusion accurately. This involves eliminating non-intrusion events caused by operating environments from intrusion events. To address the issue, this paper states the tensile strain principle of optical fiber and combines autocorrelation with frequency-energy distribution of signals to extract vibration characteristics. Finally, an identification technique is presented based on vibration theory to distinguish events. The results of simulation show that the algorithm can eliminate vibration characteristic of non-intrusion events so as to minimize the nuisance alarm rate and make prepare for the following recognition of intrusion.