Abstract: Instantaneous rise of load during the initial pressure/limit pressure switching process of a steam turbine occurs in a thermal power plant. From the logic of the initial pressure/limit pressure switching and the change of the main parameters during the switching process, the reason is that the pressure controller command rises moment and the speed load controller command decreases moment, and there is a time interval of about 1 s, causing the high pressure regulating valve command to rise. The opening of the regulating valve reaches 80% instantaneously, causing load fluctuations. After the delay block in the pressure controller logic is changed, the high pressure regulating valve is stable during the switching process.