The instrument landing system provides important approach guidance for aircraft, ensures the stability and reliability of glide signal, and keeps the aircraft approach smoothly, which guarantees operation safety. When the aircraft is unstable during approach, the crew should promptly feed back to the CNS technical supporter through the controller. In case of an abnormal condition is reported by multi crew, it shall be systematically sorted out and investigated to ensure operation safety. There are many factors causing aircraft approach instability, which may be abnormal spatial signal structure caused by abnormal amplitude or phase, non-compliance with requirements of siting, radio interferences, or wrong interception of signal. Researching on an instability of glide signal reported by multiple flights in an airport, the instability of approach caused by aircraft intercepting glide signal prematurely due to procedures, operations and other factors is analyzed, and relevant improvement methods and mitigation measures was put forward.