Acoustic resonant cavity is an important element in photoacoustic absorption spectroscopy. The spherical acoustic resonant cavity is widely reported to have the advantages of small volume, high-quality factor, and easier improvement of gas absorption path compared with a traditional cylindrical acoustic resonator. To further reduce the cavity volume and realize the miniaturization of the cavity, this paper first reports a gas sensing system using photoacoustic spectroscopy based on a hemispherical acoustic resonant cavity with a radius of 15 mm and a volume of 7.07 mL. The frequency response of the cavity was characterized to optimize the operating frequency and microphone position through theoretical simulation and experimental verification. To further improve the performance of the photoacoustic spectroscopy gas sensing system, the gas absorption path is further enhanced by combining with a multi-pass cell by increasing the light inlet dimension of the acoustic resonant cavity. The multi-pass cell comprises two mirrors with a diameter of 25.4 mm, a radius of 100 mm, and a distance of 20 cm, which results in a light spot of single line mode on the mirror surface. The photoacoustic signal amplitude of methane and formaldehyde were enhanced by 6 and 4 times, respectively. Using a multi-microphone method, the photoacoustic signal was increased by four times. The developed photoacoustic spectrometer was coupled to two tunable lasers emitting at 1 653 and 3 640 nm, which were irradiated into the cavity in two opposite directions to realize dual-gas detection (methane and formaldehyde) with measurement sensitivity of 2.11×10-6(methane, 20 s) and 0.71×10-6(formaldehyde, 20 s), respectively. The photoacoustic spectroscopy system was calibrated by measuring different methane concentrations, which showed a good linear relationship between photoacoustic signal and methane concentration. Allans standard deviation method was used to evaluate the instrument stability of the photoacoustic spectroscopy system under long-term operation. The methane detection sensitivity was enhanced to be 0.4×10-6 with an optimal integration time of 660 s. The hemispherical acoustic resonant cavity, due to its good light source applicability and small volume et al., can be employed with good application prospects in the field of gas sensing using photoacoustic spectroscopy.