Abstract: The unit's operating characteristics of power load unbalanced protection (PLU) and over-speed protection control (OPC) after load rejection and rotor overspeed was simulated. And the impact of overspeed protection action on the unit's thermal system was analyzed. The results show that: when the unit's power load unbalanced protection and rotor overspeed protection act, shutting the high-regulation valve and the intermediate-regulation valve quickly will cause the main steam pressure in front of the valve rising to 1.1415 times of the rated pressure rapidly. A certain impact force can take place on the valve of the unit, causing overpressure of the pressure-bearing components and the action of the safety valve, and ultimately leading to unstable combustion of the boiler. The only advantage of the power load imbalance protection is that, the unit can implement overspeed protection as early as the speed of the unit, but the two cases are all effective in the overspeed control. In comparison, the malfunction is more harmful.