The low absorption intensity of gas in the near infrared band is not conducive to the measurement of trace gas. A high-sensitivity and high-precision laser spectroscopy system for the simultaneous detection of atmospheric multi-component greenhouse gas was established based on the fundamental frequency absorption properties of molecules in the mid-infrared band, which uses a single novel room-temperature continuous-wave quantum cascaded laser (CW-QCL) combined with wavelength modulation spectroscopy (WMS) and a long-path optical-absorption cell. The output wavenumber of the system ranges from 2202.8 cm ^-1 to 2205.6 cm ^-1, covering the central absorption spectra of CO, N₂O, and H₂O. The results show that the measurement precisions of 1.83×10 ^-8, 1.86×10 ^-9, and 1.19×10 ^-4 for CO, N₂O, and H₂O, respectively, at 1 s time resolution. The minimum detection limits of the system can be further improved to 1.8×10 ^-9 for CO, 0.16×10 ^-9 for N₂O, and 1.5×10 ^-5 for H₂O by increasing optimal integration time to 100 s. The long-time measurement and analysis show that this system is simple in components, easy to use, and suitable for long time measurement of atmospheric multi-component gas. It can be widely used in the fields of atmospheric chemistry and greenhouse gas for highly sensitive detection research.