梯形明渠临界水深实质上是求解含一个参数的一元六次方程，理论上无解析解，现已有简捷、准确、通用的直接计算方法；复式梯形断面明渠临界水深实质上是求解含两个参数的一元六次方程，理论上更无解析解。在梯形断面临界水深研究的基础上，经过数学变换，并应用迭代理论，提出了复式梯形断面明渠临界水深的简捷、准确、通用的直接计算公式，最大误差为1.49%，能满足生产实践的需要。

In fact, the formula for calculating critical depth of duplex trapezoidal is to deal with a formula which has two parameters whose highest power is six, but in theory there is no answer. This article introduces a short- cut, correct and common- used method for calculating critical depth of duplex trapezoidal channel obtained from iteration theory. The maximum error of calculated results is 1.49%. This method has the advantages of applying to a large range and simplicity.