A mid-infrared gas detection system based on barium gallium selenide crystal (BaGa4Se7) optical parametric oscillation (OPO) technology is developed. The BaGa4Se7 OPO pumped by a commercial 1 064 nm pulsed laser is used to achieve 3~8 μm tunable output. The system is further used to detect air, carbon dioxide and exhaled gas, and the change of mid-infrared energy near 4.2 μm is studied when the temperature and pressure changed. In addition to the above gas detection, the system can also be used to achieve more gas detection, calibration, as well as biological, environmental and other aspects of the research.