Abstract: The parametrically sensitive point of a reactor was proved to be an inflection point of the hot spot locus by taking the definition of parametric sensitivity as the theoretical basis and mathematical starting point. The upper limit of the safe operation temperature was derived with respect to a one dimensional paeudo - homogeneous plug flow reactor with constant wall temperature, in which a first order reaction was carried out. With regard to the relation between the hot spot on the critical operation curve and the inflection point on the supercritical one, a criterion of the safe operation of reaction systems is obtained. This formula agreed well with the experimental data of El - Swai et al.