To improve the applicability of methane gas detection in complex environments, a hollow-core band gap photonic crystal fiber is used as the optical chamber,realizing the inserted detection of homologous methane concentration. Tunable diode laser absorption spectroscopy with a 0.5-m hollow-core band gap photonic crystal fiber is used for the online detection of methane gas. The detection limit of the designed detection system is approximately 1.92×10 ^-5 and the long-term stability of detection fluctuation is less than ±2.18%. The single-ended total reflection design and homologous detection method make inserted detection possible in complex environments. This work provides the research foundation for distributed detection using a single light source.