Abstract: The traditional finite element method (FEM) involves constructing mesh by hand,and the computation is complex when the analysis relates to seepage with free surface. In this paper, the finite difference method (FDM) based on the boundary-fitted coordinate (BFC) transformation is presented. The curvilinear grid system, with computational boundary coincident with the physical boundary, is numerically obtained by solving the Poisson equation. The seepage analysis is then done by FDM in a uniform transformed Orthogonal Coordinate system, and a uniform computer code is compiled which can be consistently applied to problem in domain of any shape. Case studies showed that the method could get satisfactory results and was more efficient than traditional FEM.