The calculation formula of normal water depth in the standard type horseshoe crosssection presents shortcomings such as complicated formula and cumbersome process. To overcome this, here we mathematically transform the fundamental equations of uniform flow in the standard type horseshoe crosssection tunnels. By determining the range of the formula according to the hydraulic tunnel design specification and engineering application, we obtained a simple and feasible calculation formula for normal water depth in the standard type horseshoe crosssection tunnels based on the principle of optimization. Results showed that the maximum relative error of normal water depth was 0.585％ in the utility range, and the relative error over 95％ of the whole range was less than 0.20％. The proposed formula satisfies the needs of engineering practice due to its high accuracy and simple form.