Abstract: Output charactrisitics of air-cored pulse transformer composed of tapered spiral winding are theoretically analysed. Parameters in the equivalent circuit are calculated to optimize output voltages. A compact spiral air-cored pulse transformer with high transformation ratio is built, which can be used on ultra-wideband system.The transformer can be charged by commercial power instead of high voltage source. The experimental result indicates that the output voltage on PFL reaches 350 kV when input voltage of charging capacitor is 800 V, with a tranformation ratio of 1∶700 and a coupling coefficient of 0.52. The result agrees with that of calculation．