A case of the unit's abnormal outage under the situation of the manual mode operation of excitation regulator was analyzed.Based on the analysis,the relationship between active power and reactive power as well as their trend under the situation of the manual mode operation of excitation regulator were discussed.The accuracy of the analysis has been proved through a simulation which reproduce the fault process.The simulation has also verified the setting value of excitation-loss protection for generation trip.The analysis results in this paper will have important reference value for the operation and maintenance of power plant.