The purpose of the paper is to explore connection mechanism between interval time and pressure at draft tube inlet in successive load shedding case. Based on transient flow theory and method of characteristics, numerical simulation calculation of transition process in the successive load shedding case for pumped storage power station with layout of two turbines and one tunnel was carried out. The relationship between flow rate, running track of turbines and minimum pressure at draft tube inlet was analyzed. The result shows that there is a certain most unfavorable interval time making the time-domain asynchronous degree of first wave flow rate in two turbines maximized and the pressure at draft tube inlet is the minimum. At the same time, running track point of the first load shedding turbine is near the upper bending point in the inverse S-region of characteristic curve and running track point of the last one is near the lower bending point. The conclusion can provide a reference for calculation of extreme value of pressure at draft tube inlet in the pumped storage power station.