Abstract: Groundwater residence time is quantitatively determined with environmental tritium and an isotopic mathematical model in Qinling extra-long tunnel area. The mean age of all the groundwater is over 43 years, and even up to 70 years in the ridge zone, obviously older than that of shallow groundwater aged only 1 to 5 years. This shows great differences between the two areas in dynamic features. The age of the deep groundwater in slop zones trends to increase towards the tunnel entrance and exit, which indicates that the fissured water-bearing medium is characterized by hydraulic continuity within the tunnel area. This property is of great significance to rationally building the mathematical model for prediction of the waterflow to tunnel.