The detection of dissolved acetylene (C₂H₂) in transformer oil is an important means to detect the operating state of transformers. In order to realize the low concentration of dissolved acetylene gas in oil, a detection system of dissolved C₂H₂ gas in oil based on a miniature resonant Photoacoustic (PA) cell is designed. The oil sample was put in a vacuum oil tank. Then the dissolved gas in oil diffused into the vacuum part of the oil tank. Because of the diffusion of the dissolved gas, the vacuum of the oil tank decreased. The gas was pumped into a gas cell after a set time. The gas in the gas cell was pumped into the detection system by a gas pump to obtain the concentration of C₂H₂. The degassing time was about 40 min. A Distributed Feedback (DFB) laser was selected as the excitation source of the PA signal. According to the spectrum data of C₂H₂ obtained from the HITRAN database, the central wavelength of the DFB laser was chosen to be 1 532.83 nm with an emission spectrum width of 0.3 pm after considering the cross interference of H₂O and CO₂. The output power of the DFB laser was 13.2 mW. To keep the laser power stable, the temperature of the DFB laser was kept to be 27℃. The output of the DFB laser was modulated by a sine signal which was produced by a signal processing circuit of a Field Programmable Gate Array (FPGA). The power of the laser was amplified to 200 mW by an erbium doped fiber amplifier before the laser was emitted into the PA cell through a fiber collimator. The volume of the PA cell was 12.4 mL. In the PA cell, the laser was reflected by a gold-plated mirror with 98% reflectance to double the absorption length. According to the principle of the PA, the PA signal was increased with the increase of the absorption length. A constant temperature device was used to stabilize the temperature of the PA cell at 30℃ to prevent the resonant frequency of the PA cell from drifting due to temperature changes. An electric microphone was used to detect the PA signal. The responsibility of the electric microphone was good between 10 Hz and 10 kHz. A lock-in amplifier based on FPGA was used to extract the second harmonic signal from the electric signal of the microphone. The response frequency range of FPGA lock-in amplifier was 1 Hz~10 kHz. Finally, the electric signal was processed by a LabVIEW program to obtain the concentration of dissolved C₂H₂ in oil. The signals of 50 μL/L standard C₂H₂ gas was recorded during the frequency range from 1 000 Hz to 3 000 Hz. The maximum value of the signals was obtained at the frequency of 2 565 Hz. Because of the use of second harmonic technology, the resonance frequency of the PA cell was 5 130 Hz. Modulation current and bias current were determined to be 5 mA and 102.8 mA respectively at the resonance frequency. Dissolve different volumes of C₂H₂ standard gas into transformer blank oil to prepare oil samples with concentrations of 2 μL/L, 10 μL/L, 20 μL/L, 40 μL/L and 50 μL/L. Under the condition mentioned above, the responsibility of dissolved C₂H₂ in oil was 1.475 μV/μL?L-1. The performance of the system was verified by repeated experiments on dissolved C₂H₂ in oil samples of various concentrations. And the system had good repeatability with the absolute error value of each test within ±0.2 μL/L. The minimum detection limit of the system for dissolved C₂H₂-in-oil samples is 0.2 μL/L. The detection system realizes an ultra-high sensitivity measurement of dissolved acetylene gas in oil, which can provide a new technical solution for on-line monitoring of dissolved gas in transformer oil.