Abstract: For a unit overload accident at Shangtong Dam Power Station, the unit overload accident was analyzed by finding the data before and after the accident. It is found that the overload of the unit is due to the frequent adjustment of the unit load, so that the unit load has not been adjusted in place, the active adjustment timeout causes the active adjustment of the unit to exit, coupled with the inertia of the water flow of the long diversion tunnel, the water pressure of the volute inlet of the unit rises rapidly, so that the unit load rises rapidly, and the LCU program logic of the unit is defective, when the active power value of the unit exceeds the program set value, the active adjustment of the unit will automatically exit and cannot be re-entered, causing the unit to be seriously overloaded for a long period of time. By modifying and improving the LCU program of the unit, the hidden danger of not being able to carry out active adjustment when the unit is overloaded is eliminated.