Abstract: This paper presents photoneutron group constants of beryllium and the neutron source strength value in the core on refueling period and before the critical transition. The paper set up the analytic function model, given neutron source strength value and the time of which the core achieve the periodic protection when the core inserted the different reactivity rate between physical startup to the critical, and given reactor power changes with the time and the total released energy and other physical parameters when the core happened supercritical delayed transient and prompt transient at introducing step reactivity.