The seepage model in FLAC3D was extended using its in-built programming language FISH. The SWCC (Soil Water Characteristic Curve) for seepage in unsaturated loess was best-fitted using MATLAB. The obtained parameters were written as a subroutine and invoked in each step in the seepage model of FLAC3D to simulate the unsaturated loess. One-dimensional infiltration problem in three boundary conditions was modeled using both the finite difference method and the finite element method to verify the extension in FLAC3D. The comparison results show that the above method is appropriate. The findings of this paper are meaningful in the investigation of the unsaturated soil theory and in the application of FLAC3D to simulate unsaturated soil problems. 