Abstract: At the first,the fundamental theory and characteristic of the flow nets of 2-D. Darcy's seepage flow problem are introduced. Based on the variational principle and finite element method,the numerical method and the corresponding calculation expressions are presented in detail. Three illustrative examples about the numerization and drawing of the flow nets of seepages through embankment and sluice foundation are successfully given. The method has foundamently soluted the problem that all of the complicated flow nets in geoengineering may be numerically drawn with quick and high precision.