Abstract: The effective stress principle is the basic theory of soil mechanics. It is of an important status, but its derivation is debated among the scholars. In order to disclose the physical and mechanical essence of the effective stress principle, explain the basic concept, and clarify the incorrect comprehension, taking the saturated soil body as an analytical object, the equation of effective stress principle under gravity and uniformly distributed stress is deduced by using the “adequate rising force” method. It is analyzed that the common conception of “ignoring the solidphase contact area to deduce the effective stress equation” is unsuitable. According to the essence, a simple method is given to amend the effective stress equation in a saturated rock or concrete, and the amended equation is in accordance with the equations deduced by former researchers.