Abstract: 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.