The exit of Qinling Water Diversion Tunnel is located nearby Jinpen Dam at Heihe Valley in Xi’an, China. In order to shorten the length of the tunnel line and reduce the engineering investment, the geological condtions of the tunnel section crossing underneath Jinpen Dam are studied. The influences of engineering geological conditions, hydrogeological conditions and the construction of the tunnel section of Qinling Water Diversion Tunnel on seepage of Jinpen Dam are analyzed in detail by means of comprehensive investigation method and qualitative analysis and quantitative calculation method. Finally, some suggestions are given. The construction practice shows that the tunnel construction scheme is rational and effective,and just has a liltle impact on the environment of Jinpen Dam and grouandwater.