The critical water depth in common city-opening shaped crosssection needs to solve the high order implicit function equation. There is no analytical solution theoretically. The graphical method or trial and error algorithm is complicated and time consuming. This thesis obtains an approximate formula calculating the critical water depth by constant deformation on the basic equation of the critical water depth in cityopening shaped crosssection and introduces dimensionless critical water depth according to optimization theory. The error analysis and examples manifest that the maximum relative error of critical water depth is less than 0.39% within the common project and the formula has a direct and simple form, high precision and? widely applied range.