A novel on–off-beam quartz-enhanced photoacoustic spectroscopy (OOB-QEPAS)-based trace gas sensor was proposed for the first time (to the best of our knowledge). The on-beam and off-beam acoustic amplification modes were realized in this sensor simultaneously. The OOB configuration consists of two T-shaped resonant tubes. By using three fiber collimators and a right-angle prism, a laser beam was controlled to pass through the branch tube and main tube of each T-shaped resonant tube, respectively. The sound field distribution was calculated by using the finite element analysis method, which provides a numerical reference for experimental research. Water vapor (H 2 O) was chosen as the target to investigate the sensor performance. The geometric parameters of the OOB configuration were optimized. Compared to a conventional bare-QTF-based QEPAS system, the OOB-QEPAS technique achieved a 26.6 times improvement in 2 f signal amplitude.