Abstract: An air core pulse transformer is specially designed to provide the high power electron beam and required magnetic field for the high power microwave device simultaneously. First, the primary coils produce the changing magnet field after the primary capacitor discharges. Then a high voltage pulse is induced in the secondary coils to charge the Pulse Form Line (PFL). The magnetic field produced by the primary coils can keep a very high value when the voltage on PFL reaches the required value (not the highest value). Finally, the main switch is triggered when the required value of the voltage on PFL is reached. So the power supply can provides the electron beam and the required guiding magnet field for microwave device simultaneously when it is properly designed.