To explore the high storage capacity of hydrogen storage material, the structural feature and hydrogenated mechanism of Li₄BN₃H₁₀ alkali metal hydrides are studied by using the first-principles calculations. The calculated result predicts that the Li₄BN₃H₁₀ and Rh₄BN₃H₁₀ are thermodynamic and dynamical stabilities according to the phonon dispersion and thermodynamic model. Essentially, the hydrogen storage mechanism of AM₄BN₃H₁₀ hydride mainly depends on the formation of [BH₄] group and [NH₂] group. Compared to LiBH₄, the hybridization between the B atom and H atom in [BH₄] group and between the N atom and H atom in [NH₂] group can store a lot of hydrogen. However, the dehydrogenation of AM₄BN₃H₁₀ hydride prefers to [BH₄] group rather than the [NH₂] group. The narrow band gap is beneficial to hydrogen release in AM₄BN₃H₁₀ hydride. Therefore, the Rb₄BN₃H₁₀ has better hydrogen release properties in comparison to the Li₄BN₃H₁₀ and Na₄BN₃H₁₀. Therefore, we believe that the AM₄BN₃H₁₀ hydride is a promising hydrogen storage material with the high hydrogen storage capacity.