A compact off-axis cavity-enhanced absorption spectroscopy system with open-path configuration is developed by using a narrow-band CW laser as light source. The open-path configuration can avoid absorption cell wall losses and sampling induced artifacts. The absolute reflectivity of the cavity mirrors is determined to be 99.8% by a simple and effective method, and an effective optical path length of 315.66 m is realized based on a 45 cm long high optical finesse open-path cavity. In order to confirm the ability of the system, a series of measurements of CO2 concentrations are performed by targeting a CO2 line at 5001.49 cm−1. According to the time series measurements of ambient CO2, a sensitivity is 7.819 ppm while the averaging time is 1 s, and an optimum sensitivity of 193 ppb can be achieved with an averaging time of 300 s. And two 24 h’ field continuous measurements are performed to demonstrate the stability of the reported system.