Abstract: Aiming at the characteristics of multiple working conditions,long life,high reliability and extremely small samples of LIPS-300 ion thruster for asteroid exploration,a reliability evaluation method based on belief reliability theory is proposed.This theory is suitable for the reliability assessment under the uncertainty of performance degradation of the current ion thruster in service.Firstly,the Function,Performance and Margin Analysis (FPMA) of ion thruster is carried out,and the key performance parameters and performance thresholds of ion thruster are identified and determined as electron back-streaming;Secondly,the margin equation,degradation equation and reliability measurement equation of ion thruster are established based on FPMA;Finally,in order to verify the accuracy of the reliability evaluation model,based on the comprehensive consideration of the ground test technical conditions and background tasks,a test scheme for the key failure mode,namely electron back-streaming,is designed.