The factors, especially the photoacoustic cell′s size, which have effects on the amplitude of the photoacoustic signal, are analysized. When a multipass cylindrical cell with optimal dimensions is operated in the first-order radial resonance mode, the signal-to-noise ratio of photoacoustic signal can be greatly improved. Using a Ti:sapphire ring laser as the light source, a laser photoacoustic spectrometer reaching a sensitivity of 8.5×10-9 cm-1 (SNR=1:1) is realized; then the sixth stretching overtone spectrum of AsH3 was first recorded with this instrument.