Abstract: In order to improve the application scope of water depth formula at contracted section, firstly by simplifying the basic water depth equation of contracted section in rectangular channel, we obtain the non-dimensional depth iteration equation, then replacing the cubic equation by simple unary quadratic equation and taking the root of unary quadratic equation as the initial value of the iteration equation, so the water depth calculation equation can be obtained consequently. It shows that when the non-dimensional depth at contracted section is less than 0.6, the maximum relative error is 0.15%. This equation has simple form, high accuracy and wide application range, which provides a new calculation method of water depth at contracted section under special conditions.