Abstract: The characteristics of harmonic cluster of output voltage, output current and time harmonic magnetic potential of AC-AC frequency converter is analyzed. The running noise of motor affected by the characteristics of harmonic cluster is pointed out. An improved Mallat algorithm is put forward. The running noise is analyzed by the wavelet analysis using the algorithm, calculating time and the using memory of the improved algorithm is less than that of the standard Mallat algorithm by 49.6%. The frequency band energy method based on the wavelet analysis is used for getting the characteristics of the running noise. The frequency band energy of the running noise in 86-172 Hz increases conformably as the load of motor increases. The above frequency band energy can be acted as the main characteristics for reflecting the load change of motor.