Abstract: A simple model was developed to investigate the generation mechanism of virtual cathode oscillation microwave based on space-charge effect under small signal condition. With this model, the generated microwave frequency in a vircator an be calculated. On the basis of theoretical analysis, an improved virtual cathode oscillator was brought forward. In this configure, a resonant cavity is located behind anode, the cavity is very small so that virtual cathode couldn't be formed in it, the reflected beem current may excited a cavity mode, which modulates injected beam; and microwave is extracted in coaxial structure.2.5D PIC numerical simulation was conducted to study it. Microwave pulse power of 1GW at 3.66GHz for 520kV and 12.5kA injecting e-beam was obtained, power efficiency was about 15.3