To study the overheat mechanism and risk of valve saturated reactor, the technical characteristics and working principles of valve reactors are analyzed, and an approximate calculation method for core temperature rise is proposed, and by fitting with temperature rise test data under actual operating conditions, a mathematical model of saturated reactor core temperature rise is developed. Based on the results of faulty saturated reactor test and disassembly inspection, it is concluded that the manufacturing process defects will cause the heat concentration in the core during operation, which accelerates the aging of insulating materials. The simulation analysis of the saturable reactor combustion shows that the flame, smoke and temperature of the valve reactor gradually enter the steady-state within 100 s after the reactor being on fire. The results show that the valve reactor is not easy to catch fire. Even if it catches fire under extreme conditions, the combustion diffusion rate is low after tripping.