The stabilizing influence of external reflections on gyrotron operation has been demonstrated in direct particle-in-cell modeling. A model with a temporal realization duration providing an opportunity to calculate frequency characteristics with a relative accuracy no worse than 2 × 10^–6 has been used for the first time. An almost twofold reduction in the spectrum width of a 28-GHz gyrotron has been observed experimentally after the introduction of reflections of the output signal.