According to mathematical transformation for the basic equation on critical water depth of open trapezoidal channels with spherical bed, the paper poses an iterative formula on its calculaton by applying the iteration theory. Moreover, on the basis of a mass of calculations and analyses on practical engineerings and selecting the right initial value on the optimal approaching theory, a short - cut, correct and effective method for direct calculating the critical depth of open trapezoidal channels with spherical bed was proposed. 