Abstract: ‍ ‍The radiation field of glide slope of instrument landing system is formed by the radiation field of glide slope antenna and its mirror antenna symmetrical to the ground， and the radiation field performance is greatly affected by the site near glide slope antenna. In the glide slope protection area of instrument landing system， the key factors that affect the radiation field performance mainly include weed height， terrain slope and terrain roughness. The relevant standards at home and abroad gave strict regulations on these factors， but no corresponding theoretical explanation was provided. Based on the array antenna theory， the mathematical model of M-type glide slope radiation field and difference in depth of modulation was established under ideal conditions， and then the difference in depth of modulation calculation model of the beacon was established under the influence of weeds， terrain slope and terrain roughness. The performance of glide slope radiation field under the influence of these three factors was simulated and compared with the simulation results of LAGON， a professional software developed by Ecole Nationale de l'Aviation Civile of France. The results showed that the modeling simulation results were in good agreement with the provisions of the corresponding standards and the simulation results of LAGON. The research results can provide theoretical support for the independent update or formulation of instrument landing system standards and the development of instrument landing system performance evaluation software with independent intellectual property rights.