In all-optical quartz-enhanced photoacoustic spectroscopy (QEPAS) system, a Fabry-Perot demodulation method based on operating point stabilization technology is proposed. For improving the detection accuracy of the photoacoustic signal, a tunable erbium-doped fiber ring laser is used as the detection light source and a detection technique based on the direct-current error signal is employed to lock the operating point to the quadrature phase point in the system. The experiment for detection of water vapour in the open environment is carried out and a normalized noise equivalent absorption coefficient of 1.81×10-7 W/(cm· Hz ) is achieved, which is about 1.2 times higher than that of conventional QEPAS.