Continuous and in-situ monitoring of flue gas CO is an effective way to optimize combustion efficiency and reduce pollutant emission. Tunable diode laser absorption spectroscopy (TDLAS) a trace gas detection technique with a high sensitivity, high selectivity and fast time response, based on the tunability and narrow linewidth features of diode laser. It can implement fast gas concentration measurement through an single isolated absorption line. Flue gas CO detecting system based on open-path near infrared tunable diode laser spectroscopy has been experimentally studied. Using a telecom distributed-feedback (DFB) diode laser as optical source, a rotational-vibratinal line of CO near 1.58 μm has been used for real time CO concentration measurement. Open dual-path optical configuration has been adopted in this system. Influence of optical extinction coefficient fluctuation has been avoided by background fitting and subtracting. The system detection limit is about 0.125 g/m2 (path integrated concentration), and can meet the demands of in situ measurements of combustion flue gas CO in industrial environments.