Abstract: Aiming at the impact of unexpected electric breakdown on the reliability of ion thruster in service,firstly,the failure impact and comprehensive risk assessment of unexpected electric breakdown are carried out,and then combined with the sudden failure characteristics caused by unexpected electric breakdown,a reliability assessment model of ion thruster based on impact model is established.Finally,taking the 20 cm Xe ion thruster as an example,the relationship between the reliability of the ion thruster and the service time considering the unexpected electric breakdown is calculated on the basis of collecting the historical data of the ground test.In addition,the coexistence of degraded failure caused by grid system performance degradation and sudden failure caused by unexpected electric breakdown is comprehensively considered,and the overall reliability of ion thruster is analyzed and discussed.