Optical cavity ring-down spectroscopy with a Hz-level response rate is introduced to detect trace gases in the atmosphere. By loading a 100 MHz modulated sine wave signal on the electro-optic phase modulator that generates sidebands and using the mixer to extract the first harmonic generated as the error signal by the beat frequency of the carrier and sidebands after passing through the 3 m gas absorption cell， the frequency locking of a 1 572 nm distributed feedback laser on the 6 361.25 cm^-1 hyperfine transition line of carbon dioxide gas molecule was achieved. The ring-down time of the cavity without gas and the ring-down time with gas absorption were simultaneously measured using the wavelength-division multiplexing method， and the detection limit of the system obtained was 4.82×10^-10 cm^-1 on a 330 mm optical resonator. The system has a good linear response in a large carbon dioxide concentration range， and the linear correlation coefficient is greater than 0.999. The long-term observation results of the system are highly consistent with the data from Picarro commercial instruments， and the deviation between the two is less than 1.0%. The system proved the feasibility of locking the laser frequency to the molecular hyperfine transition line using the first harmonic and applying it to the optical cavity ring-down spectroscopy system to achieve a fast trace gas detection.