Abstract: A comparison of environmental isotopes D and3H in surface water and deep underground water shows that F4, a regional fault in the northern slope of extra-long Qinling tunnel, has a poor hydraulic conductivity and there is no direct hydraulic connection between Shibianyu river flowing across fault F4and deep underground water. The calculated result with environmental isotope tritium shows that the possible amount of water inflow into the tunnel from the river will be less than 6% of total possible water amount. For this reason, there is no the possibility that water in the river flows into the tunnel through fault F4.