Abstract: Coaxial vircator with beam premodulation is analyzed by solving Green’s function, and the relationships of beamwave conversion efficiency to premodulation depth and frequency are obtained. Analysis results show that beam-wave conversion efficiency is in direct proportion to second power of premodulation depth for the same injecting beam, and there is a certain premodulation frequency corresponding to a maximal efficiency for every injecting beam and beam-wave interaction area combination. Finally, enhancing premodulation depth and choosing a reasonable premodulation frequency as efficacious ways to improve beamwave conversion efficiency are suggested．