Abstract: Cold cathode technique is a significant development direction in magnetron. The application of cold secondary-emission cathode firing by auxiliary thermionic cathode in short-millimeter and THz magnetron could break the limit of cathode geometry dimension, increase the emission ability and prolong the lifetime of magnetron. In this paper, the operation principle of auxiliary thermionic cathode is theoretically analyzed and the electron phase space under different conditions is simulated with PIC code. Based on theory and simulation, a W-band cold secondary-emission cathode magnetron with auxiliary thermionic cathode is fabricated and the experimental results show that the tube could startup successfully and the output power is 340 W at 95.913 GHz when the voltage is 14.6 kV and the magnetic field is 0.72 T.