Abstract: Relationship between steady speed droop and periodic test of emergency diesel generator in nuclear power plants is investigated.With respect to the speed governing characteristics of emergency diesel generator set used in Ling’ao Phase II nuclear power plant,modes selection are proposed under detailed analysis in different speed droop and periodic test modes which is referring to relevant international standard and power plant regulation.In normal operation of the power plant,the emergency diesel generator sets are in hot standby which is under the normal pre-lubrication and preheating.In case an exceptional event or accident happens to the power plant,power loss of emergency bus-bar will automatically start the emergency diesel generator sets promptly within required safe time,so that the safety equipments in the power plant recover the electricity power.To testify the reliability of generator sets and system,a periodic test mode is stipulated in plant regulation for the power source equipments.Meanwhile,because of the property of power source,its quality of output should fulfill relevant requirements to make downstream consumers operate normally and steadily.It can demonstrate through the investigation and research that the periodic test mode can be considered and selected combining the equipment characteristic and maintenance strategy so as to heighten the reliability of equipments and further ensure the security of power plant.