Abstract: On the basis of research of current fault diagnosis method of AC-AC frequency-conversion speed-regulation system,the paper proposed a fault diagnosis method of AC-AC frequency-conversion speed-regulation system with improved wavelet packet algorithm.It expounded principle of improved wavelet packet algorithm on the basis of analyzing principle and disadvantages of standard wavelet packet algorithm and introduced the method using the improved wavelet packet algorithm to pick up characteristic quantity of fault signal of frequency converter in details.The simulation result showed that amount of calculation and appropriative storage space of the improved wavelet packet algorithm are less than that of the standard wavelet packet algorithm.Compared with current fault diagnosis method,the fault diagnosis method of frequency-conversion speed-regulation system based on the improved wavelet packet algorithm has advantages of high accuracy,fast diagnosis speed,etc.