The invention relates to a method for converting aluminium in Bayer red mud from a hydrate garnet phase to an alumina phase and belongs to the field of a thermometallurgical technology. The method comprises the following steps: firstly drying Bayer red mud, grinding the dried Bayer red mud into a fine powder with granularity being 80-200 meshes, adding a hydrocarbon organic binder which accounts for 2-8% of mass of the fine powder into the fine powder and uniformly mixing, and preparing round particles with particle size being 1-5 mm by manual kneading or a granulator; and carrying out microwave roasting on the round particles for 20-100 min, and finally carrying out thermal insulation under the condition of 400-800 DEG C for 10-60 min so as to obtain Bayer red mud round particles converted from the hydrate garnet phase to the alumina phase. The method has advantages as follows: temperature is low; reaction time is fast; and no reducing agents are required. Thus, energy consumption is low; cost is low; and defects in the prior art are overcome.