Abstract: The operating process of the multiwave Cerenkov generator with a resonant cavity is simulated when being filled with He gas at various density by PIC method. The generation mechanism of the plasma and its effects on the output microwave are analyzed. The results reveal the plasma is produced owing to the collision ionization of the He gas filled in the tube by the relativistic electron beam and its avalanche ionization effect. Due to the improvement of the beam transporting and bunching by the ions tunnel, the starting oscillation time can decrease as well as the energy conversion efficiency be enhanced when increasing the He gas density in a certain range; increasing the He gas density further can result in adverse effects, even induce pulse shortening phenomenon.