In this paper, based on optical feedback theory and V-shaped cavity-enhanced absorption spectroscopy, combined with quantum cascade laser, a detection system for CO was built up. The optical feedback effect, even and odd modes effect, and threshold current lowering effect were studied. The effective reflectivity of 99.979% and the effective absorption path of 4.48 km in a cavity with physical length of 47 cm were obtained. By adjusting the length of V-shaped cavity automatically, the spectral resolution was improved to 0.001 1 cm^-1 from 0.005 3 cm^-1. With the best integration time of 53 s obtained by allan variance, this optical system allows detection for the R(6) band of CO with high accuracy of (97.79±0.07)% and the detection limit of (0.49±0.04) ppb at pressure of 40 torr and temperature of 20 °C. The effects of gas pressure on the absorption peak value, detection accuracy and detection limit for CO were also analyzed and demonstrated.