Subnanoporous mayenite electride [Ca24Al28O64]4+(O2-)2- x(e-)2 x (C12A7:e-) as the first room temperature-stable inorganic electride has attracted intensive attention because of its fascinating chemical, electrical, optical, and magnetic properties. However, it usually needs to be synthesized through a complicated multistep process involving high temperature (e.g., 1350 °C) precrystallization, severe reduction (e.g., 700-1300 °C for up to 240 h in Ca or Ti metal vapor atmosphere), and postpurification. Herein, a facile one-step aluminothermic synthesis method was developed for the massive production of C12A7:e- powders directly from a mixture of cost-effective CaO, Al2O3, and Al powders under much milder conditions (e.g., calcination at 1100 °C in flowing Ar for 8 h). By merely adjusting the amount of Al, the electron densities ( Ne) in the as-synthesized C12A7:e- can be optimized up to 1.23 × 1021 cm-3, covering the insulator-metal transition (MIT). The further mechanistic studies of this new aluminothermic synthesis process revealed that the Al performed dual-functional roles, which not only acted as an in situ reducing agent but also dramatically decreased the formation temperatures of the mayenite structure. After suitable Ru loading, the Ru/C12A7:e- catalyst from massively produced electride powder showed a promising preliminary performance of NH3 synthesis (2.8 mmol·g-1·h-1) under mild conditions (1 atm and 400 °C).