Abstract: In the process of hydraulic transition, the highest water level determines the design top elevation and the lowest water level determines the design bottom elevation of the pump station forebay. If the designs are not correct, the forebay will overflow or be empty. Determining the extreme water levels of the forebay is of great significance to ensure the safe operation of the water supply project. Based on the KBM method, the explicit formulas for calculating extreme water levels of forebay were derived for the nonlinear dynamic equation describing the change of water levels in the pump station forebay of long distance water supply system. At the same time, the precision of the explicit calculation formulas were verified by numerical calculation, and the sensitivity analysis of the precision of formulas was carried out. The results shown that the relative error between the explicit formula of highest water level and the MOC numerical simulation result was less than 1%, and the relative error between the explicit formula of lowest water level and the MOC numerical simulation result was less than 2.5%. The presented formulas have simple forms and are convenient for engineering design.