A low-frequency photoacoustic (PA) sensor based on Parylene-C diaphragm for micro-leakage gas detection is presented. The overall structure of the sensor head includes a cylindrical brass shell, a Parylene-C diaphragm, and a PA cavity regarded as a Fabry-Perot (F-P) cavity as well. The volume of the PA cavity is only 74 μL. A distributed feedback (DFB) laser is used as an acoustic excitation source. The PA pressure signal is obtained by measuring the deflection of the Parylene-C diaphragm using fiber-optic white-light interference (WLI) method. The PA sensor head is used for acetylene (C<sub>2</sub>H<sub>2</sub>) detection. A detection limit of 36 parts-per-billion (ppb) is achieved when the lock-in integration time is 1 s.