Abstract: During the commissioning test of Unit 1 of certain nuclear power plant, the problem of insufficient water supply capacity of the bypass control valve of the main feedwater flow control system appeared. Combining with the principle of the feed water flow control and the characteristics of actual valves, this essay analyzes the root cause of this problem, and puts forward an improved method of modifying the parameters of the function controller by sections. The final test shows that the method can effectively solve the problem of insufficient water supply capacity,and make the water supply flow meet the demand of the secondary side of the steam generator.