Abstract: The ion thruster has the advantages of high specific impulse and high efficiency due to its small grids spacing and high working voltage,but it is also prone to spark discharge under the condition of changing working conditions during normal operation,resulting in unstable and unreliable operation.Therefore,it is necessary to study the influencing factors and their relations of grids ignition discharge of ion thruster.In this paper,the high voltage breakdown characteristics of molybdenum grids in ion thruster are studied under the condition of no beam.The results show that when the vacuum degree is higher than 1×10^-2Pa,the grids spacing is 0.7 mm and the grids surface is not damaged,the breakdown voltage and the breakdown electric field decrease with the decrease of the vacuum degree;when the grids spacing is 0.4 mm and the grids surface is damaged,the breakdown voltage and breakdown electric field first increase and then decrease with the decrease of vacuum degree.When the vacuum degree is lower than 1×10^-2Pa,the breakdown voltage and the breakdown electric field decrease with the decrease of the vacuum degree,and the decline rates are faster than that under high vacuum.