The discharge capacity of a tunnel drainage system generally decreases with time because of the hydraulic deterioration of the geosynthetic filter. The hydraulic deterioration of the geosynthetic drain material causes an increase in the water pressure, leading to structural damages to the tunnel lining. In this study, the deterioration mechanism of the tunnel drainage system was investigated. The stress in-crease in the lining and filter clogging were considered as the main factors resulting in the reduction of the permeability. The hydraulic deterioration was theoretically modeled considering the squeezing and clog-ging. An experimental study was conducted to identify the clogging behavior due to the sedimentation of soil particles, cement-leaching calcium oxide, calcium carbonate, and iron oxide, and validate the theoret-ical model. The predictions using proposed permeability model was generally consistent with the experi-mental results.