The precise condition of thermal runaway in microwave heating on chemical reaction with a feedback control system is quantitatively studied. First, the precise condition of thermal runaway in engineering application is derived based on the principle of the feedback control system. Then, the temperature rising rates for 4 different kinds of reaction systems placed in the waveguide and irradiated by micro-wave with different input powers are simulated. Finally, the relationship, which is inattentive so far, between the thermal runaway and the sensor’s response time is discussed. The precise condition of thermal runaway in microwave heating on chemical reaction with a feedback control system is quantitatively studied. First, the precise condition of thermal runaway in engineering application is derived based on the principle of the feedback control system. Then, the temperature rising rates for 4 different kinds of reaction systems placed in the waveguide and irradiated by micro-wave with different input powers are simulated. Finally, the relationship, which is inattentive so far, between the thermal runaway and the sensor’s response time is discussed.