梯形明渠临界水深的求解过程是求解一个单变量超越方程的过程，理论上无解析解。通过引入无量纲参数——单位水面宽度，对梯形明渠临界水深的基本公式进行恒等变形，得到计算梯形明渠临界水深的迭代公式，再与合理的迭代初值配合使用。推导出4套梯形断面临界水深的直接计算公式，其中2套计算公式印证了前人的成果，并为前人的公式推导提供了简捷、充分的理论依据。通过对多家公式形式的表述和比较，并根据精度1%和1‰的不同要求进行误差分析，结果表明：4套直接计算公式理论性强，形式简单，适用范围广，计算精度高，值得推广。

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.