The Instrument Landing System is a precision approach and landing system approved by ICAO (International Civil Aviation Organization). It consists of Localizer Equipment, Glide Path Equipment and Marker Beacon or Distance Measuring Equipment. There are three types of Glide Path Equipment antenna system, including Null-reference, Sideband-reference and M-reference, the choice of system is depended on site topography. This paper analyses the working principle of the M-type Glide Path antenna MCU of the NM7000B Instrument Landing System, and statements analysis and dispose of the fault for the 20L GP's MCU of Chongqing JiangBei International Airport, which will provide reference for similar equipment fault repair of other airports.