A system and a method of transforming a gain and an integral time constant of a proportional-integral (PI) controller according to a timer lapse time to relieve an excessive phenomenon of a steam generator level caused by rapid changes in open degrees of a downcomer feed-water valve and an economizer feed-water valve when reactor power transfers between low and high power sections. Accordingly, an excessive phenomenon of a steam generator level is relieved at a transfer time when reactor power transfers between low and high power sections. Therefore, a possibility of a stop of a reactor caused by the excessive phenomenon of the steam generator level is reduced to relieve burden on an operator and improve an operation rate and economic feasibility of a nuclear power plant.