Abstract: A coaxial vircator with a new kind of quasi- optical resonator was presented. The 3- mirror cavity's field distribution shows that the maximum field in the resonator is located in the cavity's center, which can greatly enhance the beam- wave interaction efficiency. PIC simulation of this vircator was made using the MAGIC code. When the input voltage was 600 kV, the current of the diode was 50 kA and a microwave pulse output power with a dominant frequency of 7.0 GHz, a peak power of 6 GW was obtained. The average power was 2.5 GW and its officiency was about 8.3%.