Abstract: Ultra broadband radiation(UWB) performance of microwave radiated from plasma is introduced by this paper. Based on analysis of both current waveform and microwave signal waveform received by horn antenna, some factors referring to radiation mechanism are analyzed：there are TEM and TM modals of electromagnetic waves between cathode and anode; magnetic field excited by current flowing through the plasma itself makes the waves in plasma slow waves；there are two exciting factors, one is the electrons moving with acceleration, and the other is abruptly fall of anode voltage the caused by anode spots spray. The reason of broadband radiation is that the electrons with different velocities transfer energy to the waves of corresponding phase velocity.