In order to resolve the problem of dual monitor data drift at the same time of No. 31 LOC of ILS system in Chizhou Jiuhuashan Airport, and Clearance parameters of RF fell to the lowest threshold triggered fault alarm, investigate ILS LOC transmitter system, antenna system, cable transmission line, monitor system and external environment respectively. Firstly, the transmitter transmitting power and signal waveform parameters were measured, the transmitter may exclude fault; Secondly, by replacing MCU terminal and the end of transmission line of the transmitter cabinet, and comparing the data before and after the change, excluding cable performance logic transmission failure; Finally, using millimetre to measure test voltage value of double monitoring board of the monitor system, the voltage value is all less than the standard voltage, thus, the reason of the external field environment is determined, through investigate external field, found the course lightning cable sagging, data recovery after screw the lightning cable anchor.