The invention relates to a methane gas sensing device based on a fiber-loop ring-down cavity, which is characterized by adopting a broadband light source, wherein an output signal of the broadband light source passes through an envelop filter and is modulated to generate a light pulse, the light pulse passes through the fiber-loop ring-down cavity formed from a fiber coupler, a single-mode fiber and an air chamber and is divided into two filtered waves to be output to a photoelectric detector through a dual-fiber collimator, and en explored electric signal is sent to a data processing module for processing through an acquisition card. In the invention, the measurement for methane gas concentration is converted into the measurement for the ring-down time of the fiber-loop cavity, and the time for realizing one-time measurement also only requires microsecond grade. The light pulse sequence circularly output by the fiber-loop ring-down cavity is divided into two paths by using the dual-fiber collimator, one path corresponds to a plurality of absorption peaks of the methane gas, and the other path does not absorb the methane gas. By comparing output signals of the two paths, the interference and the influence of the external environment to the sensing system are eliminated, so that the stability of the sensor is enhanced, and the measurement accuracy is increased.