Vacuum leak is a common fault in the gas power systems and helium mass spectrum is the conventional leak detection method. However, the method cannot be carried out on line and the leak could not be identified expeditiously. Acoustic emission technology is an advanced on-line non-destructive testing method. In this article, acoustic emission signal generation mechanism of vacuum leakage is illustrated, and vacuum-leak-experiments were investigated based on acoustic emission characteristic-analysis. Normalized wavelet packet band energy distribution characteristics of different leakage aperture were analyzed. The results indicate that the average time-domain waveform of the operating vacuum pump is mainly below 20 kHz; a number of high-frequency acoustic signals were inspired with the vacuum leakage; the leakage signal parameters is closely related to the leakage aperture; the normalized energy distributions of leakage acoustic emission signals are in good agreement. Acoustic emission technique is of highly practical significance for online vacuum leak detection and leakage rate evaluation.