Wavelength modulation-direct absorption spectroscopy (WM-DAS) has the advantages of both direct absorption spectroscopy (DAS) measurable absorptivity function and wavelength modulation spectrum (WMS) with high signal-to-noise ratio (SNR). In this paper, WM-DAS spectrum is used to measure the absorptivity of 4300.7 cm-1 line of CO molecule and the detection limit is as low as 4 × 10-7 (200 s) at 0.5 m optical path, room temperature and low pressure. Then, combined with a 120 m long optical path Herriott cell, at room temperature and atmospheric pressure, the standard deviation of the fitting residual error of the absorptivity function is down to ~5.1×10-5 (1 s). Finally, different concentrations of CO are continuously monitored by long-path WM-DAS measurement system, and compared with cavity ring-down spectroscopy (CRDS). The experimental results show that the measurement results of long-path WM-DAS and CRDS method are the same. The detection limit of CO concentration in long-path WM-DAS system is as low as 0.9 ppb (200s), and the WM-DAS system is simple and the measurement speed is much faster than CRDS. At the same time, the long-path WM-DAS system is used to continuously monitor the atmospheric trace CO concentration and trend for one month, and the measured results are highly consistent with those of the China Environmental Monitoring Station.