Abstract: Based on the current design of the replication loop for the steam generator level control system, after the main feed water flow regulating valve is switched from manual mode to automatic mode, the calculation basis of the level controller is the steam water mismatch signal during switching, which causes the control system to lose the feed-forward function of fast regulating feed water flow. Combined with the HH reactor trip event in the steam generator level of unit 4 in Yangjiang Nuclear Power Plant, two optimization schemes are proposed for the manual/automatic switching operation scheme and systematic design. For the optimization of the operation mode,, before the main feed water flow regulating valve is put into operation automatically, the steam water flow is manually balanced. For the optimization of the systematic design, the steps for the steam water mismatch judgment and the feed-forward compensation are added. Through the liquid level disturbance test of the steam generator in unit 4 of Yangjiang Nuclear Power Plant, it is proved that the optimization scheme can effectively improve the regulating speed of the control system and reduce the overshoot, which has an important contribution to the safe operation level of the unit.