In 1960s, "pressure-relief oblique"retaining wall was used in a water-pumping irrigation works for the first time and it is still in safe operation. The retaining wall was generalized at the area for its rational structure, cost-effectiveness and safeness. However, there are still a lot of ambiguities in terms of the calculation of forces. Analytic calculation of various forces by Coulomb Theory and Rankine Theory is given.