Abstract: The energy released （or the excursion power） at a real nuclear excursion is more than that pre-dicted by reactor kinetics because of the time delay of the first persist fission chain. In order to provide refer-ence for the development of startup procedure for the reactor and reducing the scale for the criticality accident, the energy released model at the nuclear excursion is established in this paper. Due to the time difference for the first persist fission chain occurred, the excursion power of a systems with no neutron source or only weak source is higher than that of a system with stranger neutron source, and the excursion power increases as the reactivity addition rate increases.