A plasma resonant cavity, including a cylindrical resonant cavity casing, cutoff waveguides, and a waveguide inlet circumferentially formed on the cylindrical resonant cavity casing. The cutoff waveguides are arranged at two ends of the cylindrical resonant cavity casing and employ a movable end cover structure. An intermediate through hole is formed on each cutoff waveguide with the movable end cover structure, and a raised round table is arranged on an inner end surface of the cutoff waveguide and configured with the resonant cavity.