The spectrum of the micromaser injected with A-type three-level atoms is studied. The spectrum has a Lorentzian shape and its central frequency has a shift. The effects of various parameters (for example, the atomic center-of-mass velocity, the cavity-length, the one-photon detuning, the ratio of the atomic injection rate to the cavity decay rate and the number of thermal photons) on the linewidth and the frequency shift are examined. The spectrum shows very different features in different veloc
