Recently, lattice defects (hydrogen vacancies, oxygen vacancies and nitrogen vacancies) have attracted significant attention due to their promoted activity of ammonia synthesis. Here, composites of CaBTO were prepared via treatment of Ba5Ta4O15–CaH2 mixture in a flow of H2/N2 at 600 °C for 16 h. It was found that the reduction of Ba5Ta4O15 by CaH2 significantly promoted the ammonia synthesis activity of Ru/Ba5Ta4O15. The oxygen vacancies (Ov) with trapped electrons were formed in CaBTO. Various characterizations and kinetic experiments indicated that Ov donated electrons to Ru particles to promote the activation and dissociation of N2, and accepted the hydrogen atoms from Ru surface to suppress hydrogen poisoning. The lattice N3− and H− in CaBTO could also accept hydrogen or nitrogen atoms to form NH3, which provided more reaction pathways for ammonia synthesis and further improved the rate of ammonia synthesis.