Abstract: Unexpected breakdowns of ion thruster affect its reliability under operating conditions.Upon illustrating the phenomena and consequences of these breakdowns,their main causes,including protrusions on electrode,low-pressure gases,plasma,micro-particles,and insulators,are discussed.Their mechanisms are introduced concerning field emission,low-pressure gas discharge,electrode vaporization and ionization,electron backstreaming,micro-particle impact,cathodic vacuum arc,flashover of insulators,and dielectric breakdown.A summary is given with an outlook on further study of unexpected breakdowns of ion thruster.