The microwave test method of transmission line can be applied to study the characteristics of wave absorption by plasma. With this method, how the parameters of microwave plasma generator, the medium being argon, and the vacuum environment affect the variation trends of plasma return loss, which imply the plasma jet attenuation of reflected electromagnetic wave, is studied in vacuum environment. Experimental results show that when the mass flow rate and output power of plasma generator are 52.5mg/s and 52W, respectively, microwave plasma jet can absorb the energy of reflected electromagnetic wave maximally at the frequency near 5GHz. With the output power increasing and the environment pressure decreasing, the wave attenuation of plasma will be increased. The plasma jet will attenuate reflected wave maximally with appropriate generator parameters.