Abstract: The exhaust plasma plume from the ion thruster would affect the electromagnetic wave that passes through it. To reveal the effect of 20 cm ion thruster's plume on the transmission of the microwave, tests were carried out in a special facility with a wave- transmitting vacuum tank. The test approach is to locate a pair of receiving and emission horn antennas at two ends of the tank over against to detect the microwave passing through the plasma plume. The results indicate that the amplitude attenuation is less than 4 dB and the phase shift is not more than 80° in the 1.0～5.0 GHz microwave band.