In this paper, we present the reaction mechanism facilitating ternary ruthenium complex hydrides to successfully produce NH3 at low temperature (448 K ≤ T ≤ 573 K) by selective N2 activation and escaping H2 over-saturation. This work shows the unique ability of the [RuH6] 4-catalytic center in the ternary ruthenium hydride to shift its rate-determining intermediate states and transition states of the N2+H2 to NH3 reaction path in response to the lowering of the reaction temperature, which brings a significant change in reaction order of hydrogen and ammonia. This variation in the kinetics as a function of operating conditions (temperature, reactant partial pressures, etc.) is not a common phenomenon in catalysis but is observed in some cases . Nonetheless, the mechanistic details behind it are seldom investigated, especially at an atomic level. Here, we achieve this via seamless integration of experimental and computational techniques to reveal the temperature-dependent catalytic process. Our finding discloses that an electron-rich active center with a comparable affinity towards N2 and H2 are critical for mild-condition ammonia catalysis. The thorough fundamental understanding developed in this study can be further used to design new low-temperature ammonia catalysts with better performance and has the potential to drive the green ammonia technology into a new direction.