A self-calibration fiber-optic photoacoustic (PA) gas analyzer based on 2f/1f wavelength modulation spectroscopy (WMS) is proposed, which utilizes gas and solid multipass absorption enhancement. The laser light is incident obliquely on the cell wall, and one end of the cell is equipped with a highly reflective mirror. The gas analyzer takes full advantage of the miniature multipass PA cell, which enhances the absorption of gas and solid simultaneously. As a result, the double absorption enhancement of 1f and 2f PA signals are realized. A dual-channel lock-in white-light interferometer based on fiber-optic PA demodulation is designed to simultaneously extract the 1f and 2f PA signals detected by the silicon cantilever. The experimental results of methane gas detection show that the minimum detection limit (MDL) of the PA gas analyzer is 20 ppb when the integration time is 60 s. Moreover, the detection error of gas concentration is within 3% when the laser power is reduced by half. The fiber-optic PA gas analyzer eliminates the influence of changes in the laser power and optical path loss, which can be used for the high-precision detection of trace gases.