Abstract: An on-line monitoring and control system is introduced,which can be used in all kinds of plate spring fatigue forts and breakage detection.A special acoustic emission(AE)sensor,which is easy to attach and has high interference-free ability,has ho developed based on AE signal occurring in plate spring fatigue breakage,The system is composed of a 8098 microprocessor and a spedal AE signal processing circuit.The 8098 microprocessor is programmed to carry out the crack monitoring and diagnosingThe system provides a user friendly man-machine interface and a power control signal tO the fatigue testing machine when a crack occurred in the plate spring. Experiments show that the system is highly reliable and effective.