The invention discloses a resonant cavity of a hollow-core photonic crystal fiber gas laser. The resonant cavity comprises a microwave source which is connected with a microwave resonant cavity body through an output coupling waveguide. A hollow-core photonic crystal fiber is placed in an output coupling structure in the microwave resonant cavity body. The two ends of the hollow-core photonic crystal fiber both penetrate out of the output coupling structure, and one end or two ends of the hollow-core photonic crystal fiber is or are in fusion connection with a fiber Bragg grating. According to the resonant cavity, the narrow line wide output of the hollow-core photonic crystal fiber gas laser can be achieved without additionally arranging a grating element, the Bragg grating is used as the laser resonant cavity body, the size is small, the resonant cavity is easy to adjust, and laser operation is stable.