A new monitoring method is described on the basis of the technology of the cooling tower fan monitoring and failure diagnosis, the warning system uses microprogrammed control unit real-time monitoring the blade. A magnet was fixed on the tip of every blade and these magnets were uniformly distributed in the same plane. A Hall proximity switch was fixed on the air duct which was at the same plane with the magenet, and the Hall proximity switch was used as a signal acquisition sensor, and the pulse signal of the Hall proximity swith was collected through microprogrammed control unit. Using the timing and interrupt function of microprogrammed control unit, the pulse signal was analysed and processed, when the blade breaks down and deforms, the preprogramme shut down the fan and outputted a forecast alarm, so the blade accident would be prevented. It is proved that running state of the system is good by production and practice and the monitoring method can greatly improve the reliability of the fault monitoring and warning. The monitoring system has a wide application.