The invention relates to a micropore sealant of aerated concrete blocks and a preparation method thereof. The sealant is characterized in that: the sealant comprises 29 to 39 wt% of micropore filling powder, 20 to 30 wt% of a hydraulic cementing material, 1 wt% of a plasticizer and 40 to 50 wt% of water; the hydraulic cementing material is accelerated aluminate cement or iron aluminum sulfosalt cement; the plasticizer is dibutyl phthalate or chlorinated paraffin; and the micropore filling powder comprises 30 to 60 wt% of barite powder, 5 to 35 wt% of zinc oxide, 15 to 45 wt% of mica powder, 10 to 30 wt% of talc, and 10 to 30 wt% of kaolin. The preparation method comprises the following steps: preparing the micropore filling powder for standby; putting the micropore filling powder, the plasticizer and water into a mixer according to above formula component ratio, and mixing to prepare a seal slurry; and drying the aerated concrete blocks to the water content of less than 10%, and sealing surface micropores of the blocks with painting or spraying.