A carbon monoxide (CO) measurement system based on photoacoustic spectroscopy with a 2.3 μm mid-infrared tunable diode laser is built. The CO absorption line at 4300.699 cm ^-1 is selected as the sensing object. In order to eliminate the influence of long relaxation time of CO molecule on measurement, water vapor is added into experimental gas to enhance CO photoacoustic signal. By optimizing the modulation parameters, the optimal modulation amplitude and modulation frequency of the system are determined to be 4.29 cm ^-1 and 785 Hz,respectively. Under the optimal experimental conditions, there is a good linear relationship between the second harmonic signal of the selected spectral line and the CO concentration, and the linearity is 0.994. The 2.13×10 ^-6 volume fraction of CO in the air can be retrieved from the relationship. Finally, by using the Allan variance to analyze the long-term stability of the system under dry and wet conditions, the detection limits under dry and wet conditions are 1.18×10 ^-7 and 0.58×10 ^-7, respectively. It is proved that the addition of water vapor can effectively improve the CO detection sensitivity of the system.