Abstract: This paper studies the multipactor of the satellite-borne helix antenna, where the typical spaceborne double backshot spiral antenna is taken as an example. Firstly, the multipactor is designed and the threshold of the antenna is simulated by the HFSS software, to ensure that the requirements of the 50 W power capacity of the satellite microwave system can be met. Then, the forward and backward zero detection method is used to test the power tolerance and the multipactor of the antenna to further verify the power tolerance of the antenna. In order to study the phenomenon of the antenna multipactor and its effect on the performance of the antenna, the offset power withstand test of the antenna is carried out. It is shown that when the input power increases continuously to 200 W, the multipactor effect is observed once during 30 minutes, while it does not affect the satellite applications in orbit.