We use a 1392 nm distributed-feedback laser diode to develop a highly-sensitive optical gas sensor. The sensor is based on quartz-enhanced photoacoustic spectroscopy (QEPAS). A 20 kHz piezoelectric quartz tuning fork (QTF) is employed as the pressure transducer to collect the photoacoustic signals. A 2f wavelength modulation technique is used to improve the detection sensitivity. We investigate the laser modulation depth to optimize the QEPAS signal. To better characterize the sensor performance, we measure H2O for a long time and carry out the Allan variance to demonstrate the good long-term stability of the sensor system. With the 70 s integration time, we can achieve a detection limit of ∼4.3 ppm.