Abstract: There are two methods to calculate soil-water pressure in low permeability clay, soil-water pressure calculated jointly and soil-water pressure calculated separately. Which one to be taken to apply in engineering is controversial in academic and engineering circles. The two methods can only consider the extreme case, and the results are quite different. This is mainly because both methods do not fully take into account different physical properties of soils. A unified water-soil calculation formula considering the permeability of soils is proposed. This method is appropriate for most kinds of soils from sandy soil to clays and it ensures the continuity of earth pressure calculation. The concept of the method is very clear and the calculation can be used widely. Finally, the proposed method is validated by an example.