Abstract: To improve the correct rate of fault diagnosis, a method of mechanical fault diagnosis based on complex bispectrum was proposed.This method diagnosed fault according to the different coupling modes of complex bispectrum.Because of different coupling modes, the complex bispectrum contained different coupling information of signals, and the information could be used to diagnose fault.In the fault diagnosis experiment of pressure reducing valve, the same coupling mode was applied to both fault signals and normal signals.Because two dimensional wavelet could effectively extract frequencies information from signals, the features of coupling information in complex bispectrum were extracted by 2-D wavelet, then the features were input into support vector machine to diagnose fault.Experiment results show that the correct rates of the same fault are also different with the different coupling modes.