Abstract: The discharge chamber is the most important component of the ion thruster， currently， the work of diagnosing of the plasma inside the discharge chamber has not been carried out. In this paper， the application of emissive probe in diagnosis the plasma inside the discharge chamber is introduced. The principle of diagnosing the plasma potential using the emissive probe， the diagnostic scheme and the composition of the emissive probe system are mainly introduced. The data processing method and the factors which effect diagnosing accuracy are demonstrated.