Abstract: The effects of the plasma generated from the air ionzation on the output characteristic of microwave are given. Keeping the external wave constant, the diode current increases, the diode voltage decreases, and the microwave frequency decreases with the increase of air pressure. The space charge neutralization introduced by the plasma ion and the microwave energy absorption produced by the plasma electron will affect the beam-wave interaction simultaneously. In certain range of air pressure, the advantage of space charge neutralization leads to the increase of the output power. However, when the pressure is higher, the compact plasma electron will strongly absorb the microwave energy, and lower the output power, even result in pulse shortening. On the other hand, due to the presence of the