In order to study the pressure-reducing capacity of the tunnel drainage system in the water-rich areas, a three-dimensional fluid solid coupling numerical model was established using FLAC3D software based on the Fenghuang No.2 tunnel of the Hezhou Bama Expressway (Laibin to Du′an section). A new comprehensive evaluation method for the pressure-reducing capacity of drainage system was proposed to investigate the influence of three factors including the distance between circumferential drainage blind pipes, the ratio of permeability coefficient between surrounding rock and initial support, and the height of water head on the pressure-reducing capacity. The results show that the circumferential blind pipe has a significant reducing effect on the water pressure of the lining arch ring. The water pressure at the lining arch and side wall is relatively high, only lower than that of the inverted arch. When the distance of the blind pipes is 3~5 m, the water pressure at the top of the lining along the longitudinal direction of the tunnel exhibits an "M" shaped variation trend. However, when the distance of the blind pipes exceeds 5 m, the pressure reduction effect of the blind pipes begins to significantly decrease, and the variation characteristic of water pressure along the longitudinal direction of the tunnel changes to a "flat-topped mountain" shape. The pressure-reducing capacity of drainage system was evaluated by comprehensive stress index and pressure reduction coefficient, obtaining a prediction formula for the system pressure-reducing capacity. The maximum prediction error is 8.25%, indicating a high level of accuracy. The impact of the three factors on the pressure-reducing capacity is in the following order: the distance between circumferential blind pipes > the ratio of permeability coefficient between surrounding rock and initial support > the height of water head. The larger the ratio of surrounding rock to initial support permeability coefficient is, the lower the pressure-reducing efficiency of the drainage system is. It is necessary to reduce the distance between blind pipes or reinforcing the surrounding rock with grouting to increase the pressure-reducing efficiency of the drainage system. Taking into account multiple factors and the variation pattern of pressure-reducing capacity, construction period, and cost, it is suggested to set the distance between circumferential blind pipes at 5 m for this project. The proposed comprehensive evaluation method for the pressure-reducing capacity of the drainage system demonstrates good applicability, providing significant reference for the design and construction of the waterproof and drainage system of the water-rich tunnel.