CH3NH3PbI3 hybrid perovskite attracts enormous attention as an excellent light absorbing material because of low cost, easy solution-based synthesis, benign defects, and so on. However, its band gap of about 1.55 eV is out of the best range from 1.1 to 1.4 eV for an optimal single-junction solar cell, which is adverse to further improve the power conversion efficiency. To obtain the perovskite with the suitable band gaps and tunable electronic structures, cubic (Li3O)M(BH4)(3-x)Br-x (M = Ge, Sn and Pb; x = 0-3) perovskites are systematically studied by employing density functional theory based first principles calculations. The calculated results show that the (Li3O)M(BH4)(3-x)Br-x perovskites have suitable tolerance factors and negative formation energies, tunable direct band gap from 0.97 to 2.42 eV, and small electron effective masses as well as the different charge densities for conduction band minimum and valence band maximum states. The calculated results also suggest that cubic (Li3O)Ge(BH4)Br-2 and (Li3O)Pb(BH4)(2)Br perovskites can show the power conversion efficiency of 23.12% for the loss-inpotential of 0.5 eV. Interestingly, the power conversion efficiencies of the (Li3O)M(BH4)(3-x)Br-x perovskites are first increased then decreased with the increase of Br atom.