Based on fundamental principle of saturated/unsaturated flow and characteristic of FLAC3D's showing pore pressure nephogram, this paper was aimed at obtaining steady seepage within the dam, capturing the face on which pore pressure was equal to zero, and then identified the phreatic line of the dam. Using this method, phreatic line of a tailing dam in a mine was computed. The results were compared to field data. It was indicated that, the phreatic line calculated by this method was slightly higher in the mass, but was lower near the surface of the dam. This method has the merit of being faster and simpler to obtain approximate phreatic line of dams in comparison with site exploration.