Abstract: The prompt supercritical process of nuclear reactor while inserting large step reactivity was analyzed with neutron multiplication formula including temperature feedback. Comparing with classical theory, neutron multiplication formula with temperature feedback was proved properly. The relationship between reactivity and output power was derived, and the results were analyzed and discussed, which can provide an important theory for the safety analysis of the nuclear reactor while inserting large step reactivity, such as rod ejection accident. It is found that the relationship between the number of neutron and reactivity is quadratic function while reactivity is larger than quotient of delay neutron.