Abstract: The calculation of the vertical component force of the relative equilibrium liquid on the wall by the integral method is very complicated, especially when it involves a parabolic equation. It is found that the pressure body method by geometric drawing is not only suitable for the static liquid, but also suitable for the relative equilibrium liquid. The theoretical basis and the basic method of calculation can be adjusted for both liquids. In this way, the integral calculation is transformed into the solution of the geometric volume, which can make the calculation of the relative equilibrium liquid as simple as for the pressure body.