Abstract: An on-line monitoring technique for blade crack fault is set up according to the mechanism of an acoustic doppler on-line monitoring technique,which is based on cracked blade's resonant phenomenon in turbine machinery.But the resonant phenominon can not be captured easily,because the fatigue crack expands very quickly due to the high frequency vibration of turbine blade.So that a new monitoring method based on the unsymmetry in vibration response of the cracked blade is developed,which utilizes the achivements in analyzing the vibration of the cracked blade.The program for calculating vibration response wave form of the rotating blade from acoustic doppler signals is worked out.Simulation research indicates that this new method can monitor the crack fault more effectively.