Gas chromatography analysis (DGA) is an essential component of power transformer faults diagnosis, allowing the evaluation of transformer insulation by detecting the content of hydrocarbon gases and hydrogen in insulating oil. Acetylene is an important characteristic gas for severe faults or defects, such as reactive arc discharge and high-temperature overheating discharge. To detect dissolved acetylene in transformer oil, this paper proposes a gas sensing method using hollow-core photonic crystal fiber (HC-PCF) as the gas chamber, based on the external-cavity interferometry of photothermal spectroscopy. Firstly, the principle of photothermal spectroscopy gas sensing was introduced. Secondly, three square micro holes were processed on the surface of HC-PCF using focused ion beam (FIB) technology to achieve rapid exchange of gases inside and outside the fiber. Finally, a heterodyne interference topology based on fundamental wave detection was designed, and the optimal modulation depth of the pump light was determined to be 0.16%, and the wavelength modulation range was 1530.370nm-1530.414nm, achieving acetylene sensing with a response time of 490s and a lower detection limit of 2.822ppm. performance.