Abstract: The characteristics of the unexpected breakdown of the ion thrusters in space and ground tests, under different operating conditions, and during different cumulative operating times, are comparatively analyzed. The correlations between the breakdown rate and its main influencing factors are obtained. It is shown that during the ground tests and during the whole space mission lifetime, the breakdown rate is high initially, and then decreases quickly in a relatively short period before reaching a long-term stability. The breakdown rate in space is generally one to two orders of magnitude lower than that on the ground, basically due to the breakdown-inducing factors on the ground such as the exposure to the atmosphere and the back sputtering from the vacuum facility. The grid material, the field strength, the beam current density, and the state of the sputtered deposits are among the main influencing factors that determine the essential characteristics of the unexpected electric breakdown of the thruster. The discharge extinguish events caused by the breakdown are worth particular attention and the preventive measures have to be adopted.