Abstract: The method and essentials of decreasing water content of over-wet subgrade fill are introduced by use of the principle of dewatering by electro-osmosis. Combining the electro-osmosis theory and electricity principle with successful experience of electro-osmosis practice, the design, calculation steps and estimating method of relevant parameters of the over-wet soil treated by electro-osmosis are presented, the specific calculation formula is shown, the method by some project practices is validated, and some design advices and points for attention are put forward. The method of calculating water quantity drain from over-wet subgrade fill from a macro-view is adopted for estimating parameters. The interface electric resistance is introduced and the influence from adjacent electrode is considered when calculating overall resistance of soil in electro-osmosis area. The experimental results show that the conclusions are reasonable.