A multi-mode diode laser absorption spectroscopy system is built by using a multi-mode diode laser with stable emission spectrum as light source. Combined with the long path absorption and harmonic detection techniques, concentration (volume fraction) measurement of CO is realized by detecting multiple absorption lines near 1570 nm. The experiments are carried out at room temperature and 20.265 kPa (0.2 standard atmospheric pressure). By configuring different CO-N2 gas mixtures, a series of different concentrations of sample gases are measured. The system is calibrated by using nine kinds of CO-N2 mixtures with different concentrations. The calibration formula is obtained and used to inverse CO concentration. The results show that in the case of volume fraction below 10%, the consistency between the measured and known values is higher, and the average deviation between them is 2.57%. By analyzing the mixed gas measurement signal of 0.5% volume fraction, the system detection limit of CO is 3.03×10-5. The system can meet the on-line monitoring of CO, and it has high stability and high sensitivity. The experimental device is simple and easy to use.