Abstract: As an improvement and a variant of the conventional photoacoustic spectroscopy (PAS), the quartz-enhanced photoacoustic spectroscopy (QEPAS) offers a high sensitivity, strong immunity to ambient noise, compact volume and low cost. A double-pass off-beam QEPAS system for acetylene (C₂H₂) detection was proposed. To enhance the QEPAS signal, it employed two AmRs and a right-angle prism to construct a double-pass enhancement channel. To enhance the signal to noise ratio (SNR) and stability, an optical fiber Fabry-Perot interferometric sensor with quadrature point self-stabilization was used as the sensor for the detection of quartz prong vibration and a wavelength modulation and a 2nd harmonic demodulation technology were employed. Experimentally, the parameters, including the AmR's position and the OTF structure, were optimized. The experimental results demonstrated that the minimum detection limit was ~1.99 mg·m^-3 with a 1s averaging time, which corresponded to a normalized noise equivalent absorption coefficient of 7.6×10^-8 cm^-1·W·Hz^-1/2. In addition, a good linear dependence of the 2f-signal on the C₂H₂ concentration within the range from 116 mg·m^-3 to 5 812 mg·m^-3 was obtained and the linear correction coefficient was 0.998.