Abstract: To save engineering cost, two pump-turbines could share the same main diversion tunnel at a pumped-storage station. In this case, load rejection by one turbine produces hydraulic disturbance to the other and consequently induces a load rejection of the latter, thereby resulting in a phenomenon of successive load rejection. Normally, the lowest pressure at the draft tube inlet of the second turbine will be dropping rapidly, but sometimes extreme low pressures may also occur in the first one. In this work, we studied the mechanism of rapid pressure dropping in the first turbine unit during successive load rejection based on numerical simulations. Results indicate that when the first turbine is operated at a point on the slope inflection segment of the S-shaped characteristic curve and the flow in the second turbine is increasing, this flow increase will induce a large head on the first turbine that causes a rapid pressure drop at its draft tube inlet, a situation unfavorable to this unit.