The solution process of critical depth on trapezoidal channel is to solute a single variable. There is no analytic solution in theory. With a reasonable initial interaction, by introducing a dimensionless parameter——unit surface width and doing on identical deformation of a critical depth formula on trapezoidal channel, the critical depth formula on the trapezoidal channel can be obtained. There are four sets of direct critical depth formulas on trapezoidal cross section, in which two sets of formulas confirmed previous results and provide a simple, full theory basis for the previous formula derived. The calculated results show that the four sets of direct formulas possess stronger theoretic and simple forms, may be applied to a wide range with high accuracy, and in accordance with the accuracy of 1% and 1 ‰ to the different requirements, errors of every formula are analyzed.