The invention discloses a cone angle 120 degree +/- 10 degree range-finding method of an emitter of a gamma shutdown sensor. The gamma shutdown sensor comprises an emitter and a receiver, the emitter emits gamma rays downwardly along the cone angle, the gamma rays are reflected by the ground, the receiver receives the gamma rays reflected by the ground, and the distance is measured according to relation between the distance and the signal intensity of the received gamma rays. The range-finding method of the gamma shutdown sensor is stable and excellent in the range finding performance, can be used to measure the practical distance from an aircraft to the ground more accurately, and is influenced by the morphology of the landing ground.