As a kind of important pollutant in atmosphere and hazard gas in coal mine or oil field, CO has to be monitored in real-time for the safety of daily life and industrial production. The R(6) absorption line of CO near 2334 nm, without interference of H2O absorption, is screened for a tunable diode laser absorption spectroscopy (TDLAS) system, which is used to monitor the atmospheric CO by long optical path with direct absorption techniques. With the help of off-axis parabolic mirror, an optical path of 700 m is achieved by sending and receiving light on the same path. A miniaturized measuring control system of 120 mm × 100 mm × 25 mm with power consumption lower than 5 W integrating laser driver, spectral data acquisition and processing on a single board is developed. The CO and CH4 spectra are separated on host computer by means of nonlinear fitting. By analyzing the standard deviation of spectral data, it can be derived that the system has detection precision of 10-6 at 1 s respond time. A continuous atmospheric CO monitoring experiment is carried out and the results coincide well with the CO analyzer results, which proves feasibility of the instrumentation of this system.