In order to accurately measure the trace methane (CH4) concentration in nitrogen, a continuous wave-Cavity Ring-Down Spectrometer (CW-CRDS) measuring device based on the triangular annular cavity was designed and built. The ring-down cavity is designed for autonomous processing. It consists of a concave mirror with a radius of curvature of 1 m and a diameter of 25 mm and two plane mirrors with a diameter of 12.7 mm. The total length of the optical path in the cavity is 410 mm, and the cavity material is Invar. First, we measured the baseline loss of the system, and then used nitrogen as the mixture to configure a mixture of five different concentrations of CH4 and N2. The gas was detected by the absorption peak of CH4 at 1 653.7 nm (CH4ν3 with R5). According to the absorption line characteristics, the attenuation time constant τ is calculated by the least square method using the Lorentz linear function and the CH4 concentration is calculated. The detection sensitivity of the built device to the methane volume concentration can reach 54×10-9 (540 million). Finally, the absorption spectrum of CH4 with a volume concentration of 510×10-9 in the range of 6 046.7~6 047.2 cm-1 was measured, and the measured data was determined according to the cavity free spectrum. The range (Free Spectral Range, FSR) is fitted to the τ and the absorption coefficient respectively. The obtained CH4 absorption coefficient is compared with the data in the database. The maximum error is less than 1.2×10-9 cm-1, and the highest precision is 8.8×10 -11 cm-1.