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First-principles calculations have been executed to investigate the interaction between H/He impurities and vacancy in tetragonal Be 12 Ti. The solution energies of four types of most stable H 1 V complexes are lower than that of He 1 V complexes. The binding energies of H atom to four types of vacancies are all lower than that of He atom to vacancies, indicating that He-vacancy complexes can serve as a trap to capture other impurity atoms. Besides, the formation energies of vacancy decrease when a single H or He atom is implanted into vacancy space, and the influence of He atom is greater than that of H atom. The exist of He atom in the H m HeV complexes leads to the increase of solution energies comparing with H m V complexes. In all the H m HeV complexes, the distances between any two H atoms are larger than that in a H 2 molecule, indicating that H 2 molecules can not be formed in these complexes. The trapping behaviours of He-vacancy complexes for H atoms are as follows: a HeV B e 1 and a HeV B e 3 complex can both trap up to three H atoms, a HeV B e 2 complex can capture up to two H atoms, and a HeV Ti complex can capture up to ten H atoms. The number of H atoms trapped by three types of HeV Be complexes reduces compared with that of mono-vacancy for H atoms, indicating that the retention of H atoms within the vacancy space in Be 12 Ti is suppressed by doping of He atom. The present results provide a foundational image of He-vacancy trapping mechanism for H atoms, which contributes significantly to the study on the synergistic effect of hydrogen and helium in tetragonal Be 12 Ti. [ABSTRACT FROM AUTHOR]