Abstract: To solve the problem of feed pump cavitation that may occur during load rejection process of an ultra supercritical unit due to the drop in available net positive suction head (Na) of feed pump, a mathematical model to calculate the transient characteristics of deaerator and the maximum Na decrease (ΔNa,max) of feed pump was proposed, based on theoretical analysis and experimental research on the run back (RB) transient process of deaerator. Resnlts show that comparison results between calculated and experimental data prove the proposed model to be highly accurate. To effectively reduce the ΔNa,max, countermeasures may be taken by either controlling the fluctuation of water volume in deaerator, or reducing the downcomer length under the premise keeping the mounting height of deaerator unchanged.