Abstract: With the increasing capacity of the emergency diesel generator（EDG） in nuclear power plant, it is more and more difficult to meet the requirements of rapid start-up within 10 seconds under emergency conditions. As a new energy storage technology, super capacitor has the ability of discharging rapidly in a short time which is unmatched by traditional energy storage technology. In this paper, super capacitor is designed as the backup medium voltage power supply during the start-up of EDG in nuclear power plant, so as to postpone the time required for the rapid start-up of EDG under emergency conditions and effectively improve the start-up reliability of EDG. This paper discusses the feasibility of the scheme from the aspects of capacity accounting, system scheme, equipment layout and economy of the super capacitor.