Abstract: For ion thruster,the accel grid can produce a potential barrier to stop neutralizer electrons from flowing back into the discharge chamber.The backstreaming electron current is not only a parasitic power loss,but it can damage the thruster.The back-streaming limit is determined by the "saddle-point" voltage and the factors related to the "saddle-point" including the voltage of screen grid and accelerator grid;the structure of grids;the beamlet current.In general,there are two methods to measure the back-streaming limit.This paper choose the method to monitor the increase in the screen power supply current as the magnitude of the accelerator grid voltage is decreased.