In the present paper, the authors simply describe the principle of cavity ring-down spectroscopy (CRDS) measurement technology and introduce a threshold circuit, based on 555 timer IC (integrated circuit), applied to CRDS. And we verify the feasibility of the circuit. By means of changing the input resistor and input capacity, we analyzee the influence of threshold circuit characteristics on single ring-down event, and found that the greater the input capacitance, and the smaller the input impedance of circuit, the more serious the distortion of measurement of ring-down curve, especially when the resistance is less than 50 Ω the ring-down time produces very big change. Then we collected and analyzed the C2H2 absorption signal at 6 531.780 5 cm-1 with different input resistance and input capacitance, and fitted the absorption line with Voigt profile. We found that the change of the input capacitance and resistance caused the distortion of fitting curve. Finally We give the optimal design of threshold circuit: the input impedance of the threshold circuit should be great, minimum of 100 Ω, the capacitive reactance should be as small as possible, preferably below 1 nf, at the same time it should be ensured that the threshold of the circuit time constant is much smaller than ring-down time. The experiment research on CRDS technique application has important reference value to threshold circuit design.