This work investigates the impact of cavity mirror parameters on the transmission bandwidth and sensitivity of cavity-enhanced dual-comb spectroscopy （CE-DCS） systems， starting from the principle of cavity-comb coupling. A comprehensive evaluation metric for cavity mirror performance is proposed. Based on this metric， a low-dispersion cavity mirror with a finesse of 15，000 was designed and fabricated. A CE-DCS system was developed using the designed mirror. Absorption spectroscopy experiments were conducted for CO₂ gas， and the results show that the system achieved a transmission bandwidth of 370 cm^-1 （88 % of the original comb spectrum）， with a noise-equivalent absorption （NEA） of 1.45×10^-10 cm^-1Hz^-1. These results validate the accuracy and effectiveness of the designed mirror in achieving broadband and high-sensitivity gas absorption spectroscopy measurements， providing a solid technical foundation for trace gas quantitative analysis.