On the basic of “time to ignition and degree of super criticality law”, a kind of new calculation method of scale time of adiabatic explosion under the marginally supercritical condition was worked out. Based on that and with application of Lambert W function, the function of ambient temperature and time to ignition of marginally supercritical system, under uniform and distributed temperatures were worked out. The definition and calculation of high temperature ignition reliability of energetic materials were given. Results show that the relationship between supercritical ambient temperature and time to ignition of marginally supercritical exothermic system is a function, which is decided by activation energy, frequency factor, quantity of reaction heat, specific heat, density and other physical and chemical parameters of exothermic system.