When the flow of the main feed water pump of CPR1000 unit is low, that is, when the unit is operating at low load, in order to protect the main feed water pump from damage, two low-flow protection recirculation pipelines are installed at the pressure stage pump outlet, and the two lines are installed with two minimum flow regulating valves. These two valves generally have two designs: proportional type and on-off type. The paper simulates the response of the minimum flow valve of main feed pump to water level regulation of steam generator at low load based on the full-range simulation platform of CPR1000 nuclear power unit. By analyzing the response curve of the water level of the steam generator, the advantages and disadvantages of the different valve designs are verified separately. In the proportional design, the valve position of the minimum flow valve is easy to oscillate with the fluctuation of the flow signal, which will affect the stability of the steam generator feed water control system and then affect the operation of the water level regulation system of the steam generator. At the same time, the proportional minimum flow control valve has the control dead zone. Under the transient conditions with frequent disturbance, the response speed of the minimum flow valve does not match the response of the water level adjustment of the steam generator, resulting in the unstable water level of the steam generator. In the on-off design, the stability of water level regulation will have certain advantages. After comparing the influence of the two valve designs on the water level regulation of the steam generator and analyzing the limitation of the proportional design and the importance of minimum flow valve, two improvements schemes are given for the design of the minimum continuous regulating flow valve.