Accurate acquisition of cavity ring-down signal is the basis of quantitative detection of gas concentration by cavity ring-down spectroscopy. Aimed at the characteristics of cavity ring-down signal, a cavity ring-down signal acquisition and processing system based on FPGA is designed. FPGA is used as main controller of system. The system realizes high speed acquisition of the cavity ring-down signal, accumulation average of the corresponding point and transmission of the USB data under excitation of trigger signal. Tests show that the system has good accuracy and stability (accuracy in the range of 4 per thousand). When applied in cavity ring-down system, the consistency of data from the data processing system and foreign data acquisition card can reach 99.4%. Results show that the data acquisition and processing system can fully meet the acquisition requirements of cavity ring-down signal.