Abstract: Accelerator mass spectrometry (AMS) technology developed extremely rapidly in the late 1970s. Since the substances to be detected are different, the detector is selected according to the different needs of the substance to be tested. The design and establishment of the gas detector were report in this paper. In order to establish a low energy heavy ion AMS measurement technique, a gas detector with a extreme thin entrance window was developed. The detector adopts a silicon nitride film with a thickness of only 50 nm and a membrane area of 8 mm×8 mm as the entrance window. The silicon nitride film has high hardness, thinness and uniformity, so it is an excellent incident window material for the detector. In the early stage, the detector was debugged with the 5.48 MeV ²⁴¹Am source alpha particle. And then, the detector was placed on a 300 kV small AMS system independently developed by the China Institute of Atomic Energy for ¹²⁹I particle measurement. Through simulation calculation and debugging and application of the detector. It is proved that the detector has high energy resolution, which can realize the identification of different particles well, and the measurement sensitivity can reach the international level of 10^-13, meeting the measurement requirements of low energy heavy ions.