Abstract: A cylindrical waveguide filled with thin annular plasma is studied in a strong longitudinal magnetic field. The beam and the annular plasma have been considered as anisotropic dielectric. So the hot cavity dispersion equation can be derived on the basis of the matched boundary conditions and the wave growth rate is obtained by means of equipotential line method. The dispersion equation and the growth rate with different parameters are numerically calculated. The numerical results show the Cherenkov radiation can be produced through the interaction between the former two low frequency plasma modes and the slow space charge wave, and with the increase of the in plasma density, the maser have several merits, such as enhanced output frequency , higher growth rate and significantly broader band