A novel double pass quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for atmospheric CO 2 detection is developed by use of a 1.5 μm telecommunication diode laser. A low-cost high-reflection concave mirror is positioned behind a traditional QEPAS based acoustic detection module to enhance the absorption optical path and improve the detection sensitivity. The obtained minimum detection limit is 29 ppmV for 1 s averaging time which corresponds to a normalized noise equivalent absorption coefficient (NNEA, 1σ) . With a 247 s averaging time, it demonstrates an ultimate detectable sensitivity of 1.74 ppmV.