Quantum cascade laser (QCL) is an ideal mid-infrared source for gas sensing in the wavelength range from 2.5 to 25 μm, due to its fast response, high sensitivity and selectivity for gas detecting. Prototypes of gas sensing system based on QCL have been developed by worldwide research groups. They have great potential in many applications, such as environment monitoring, space exploration, anti-terrorism and so on. The present paper gives a broad review of QCL gas sensing system, including the basic working principle, existing systems, and its application and future development.