Abstract: It is very important to reveal the evolution law of phreatic line caused by rainstorm during and after rainfall for the safety of tailings pond in operation，as heavy rainfall often leads to quick fluctuations of water level for valley-type tailings dam.Taking a valley-type tailings reservoir for example，the corresponding finite element mesh model is created，and the evolution of phreatic line under the rainfall and changing water level is studied systematically with unsteady and unsaturated seepage theory in the process of one water level fluctuation.The simulation results are shown as followings:①At the first stage of rainfall，only the phreatic line near the original tailings dam uplifts;②With the lasting rainfall，the phreatic line uplifts everywhere and the beach length keeps decreasing;③Just after the ending of rainstorm，the phreatic line continues uplifting and reaches the highest position，and the beach length becomes shortest at that time; ④In a longer time after ending rainfall，the phreatic line drops gradually and goes back to its initial position，however the moment of return of phreatic line lags longer behind the time of ending rainfall.Consequently，in order to ensure the safety of valley-type tailings dam under heavy rainfall，monitoring the phreatic line should be focused on different key items by stages according to its evolution characteristics，and especially during several days after rainfall.

Key words: Phreatic line, Heavy rainfall, Unsteady and unsaturated theory, Tailings dam