Abstract: The prompt supercritical process of nuclear reactor with temperature feedback while inserting large step reactivity (ρ0>β) was analyzed. The relationships between reactivity and time and between output power and time were derived, and the maximal power as well as the related reactivity and time were obtained. The affections of the inserted step reactivity and initial power on the prompt supercritical process were analyzed and discussed. Some useful new conclusions were drawn, which can provide an important theory for the safety analysis and operating administration of the nuclear reactor.