The grey relational degrees among different water systems of Xueshanliang Tunnel are studied by water chemical analysis experiment based on existing geological data and water gushing prediction, so as to identify the water source of water leakage. The single water source and multiple water sources are tested and analyzed. The results show that the atmospheric precipitation may be the main water source of water leakage of exit section of Snow Mountain Tunnel, and the upstream water system of Taojin Ditch and water system of tunnel exit have a high relational degree as well. Site investigation is carried out, and the results show that the atmospheric precipitation and upstream water system of Taojin Ditch are the main water sources of water leakage of Snow Mountain Tunnel. The analytical results of grey theory are proved to be feasible.