α′,β,γ-Aluminum hydride (AlH₃) is often seen alone in generation of α-AlH3 with aether. Effects of these polymorphs on the thermal stability and safety of AlH₃ were studied. At room temperature stored 650 d, the existence of γ-AlH₃ increases the decomposition rate of AlH₃ from 2.13% to 5.15%. The existence of the impurity polymorphs increases the fricition sensitivity and impact sensitivity. Furthermore, the influence to the electrostatic sensitivity is much more than the others, and the numerical value of E₅₀ is five to ten times than the that of pure α-AlH₃ polymorphs.