A synthesis method of fine-grained alpha-AlH3 is characterized in that ultrasonic catalytic crystallization is introduced in the crystallization stage on the basis of an ether process synthesis technology, and the energy, the strength, the frequency and other parameters of ultrasonic oscillation are controlled on the basis of guaranteeing the crystal phase purity of alpha-AlH3 to control the crystallization granularity of the alpha-AlH3 to be 10-80 [mu]m and realize controllable granularity. Ultrasonic assisted alpha-AlH3 crystallization is carried out to synthesize the fine-grained alpha-AlH3 with the purity being greater than 99%, and the frequency, the amplitude and the working mode of ultrasonic waves are adjusted to adjust the granularity in a range of 10-80 [mu]m. The method has the advantages of simple required device, easiness in realization of amplification, and realization of repeated and stable generation of granularity-controllable fine-grained high quality alpha-AlH3, and allows the total yield to be greater than 70%, the purity to be greater than 99% and the chlorine content to be lower than 0.2%.