Optical-feedback (OF) cavity ring-down spectroscopy consisting of a linear cavity is developed by employing a continuous wave laser diode (LD) with multi-longitudinal modes. Due to the OF effect caused by the cavity output laser back into the LD, the laser frequency is locked, and the intracavity laser intensity is enhanced. We use different concentrations of NO2 gases to test the apparatus, and the results show good agreement with theoretical values. Owing to the compactness of the laser source and high detection accuracy, the device can be used for detection of low-concentration absorbent gases in the environmental monitoring field.