Abstract: In this paper the reason why measured stress on supporting structure is generally much less than that calculated with classic earth pressure theories is explored. The method of estimating water and earth pressures on supporting structure separately and together which has become an attention and discussion focus in field of geotechnical engineering is analyzed. It is indicted that the method of estimating water and earth pressures together is inconsistent with the effective stress principal, but there might be some microscopical ground. Therefore it is an important problem deserving of further research.