Abstract: One of multi-mode ion thruster key failure modes is the character of neutralizer coupling with ion beam.The neutralizer must be operated in spot mode during its whole life.A PIC simulation code was developed to studying the electric potential distribution in the beam region when LIPS-300 ion thruster was operated in 5 kW perfect neutralization case.The electric parameter versus flow rate when ion thruster operated on 5 kW was studying through experiment，and the neutralizer keeper voltage and its oscillation versus input flow rate were studying through experiment when LIPS-300 was operated at 5 kW，4 kW and 3 kW，respectively.The optimal input flow rate was suggested to LIPS-300 that based on the result of this paper.