The invention relates to a preparation method of a sillenite bismuth silicate microcrystal, which comprises the following steps: firstly, mixing and adding bismuth trioxide powder and quartz sand into a ball milling tank, adding ethanol for uniformly mixing, and then drying to obtain a mixture; then, adding the mixture into a crucible for melting; after melting, taking the crucible out of a muffle furnace, and putting the crucible outside the furnace for quickly cooling at room temperature; and when the surface of the molten glass in the crucible is supercooled and crystallized, putting the crucible into the muffle furnace again for heating, then cooling the crucible at room temperature, and repeatedly heating and cooling the crucible to obtain the bismuth silicate microcrystal. By adding raw materials at high temperature and using a process for quickly melting at high temperature, the invention avoids the influence of uneven components caused by volatilization of the bismuth trioxide at high temperature. The prepared bismuth silicate (Bi12SiO20) microcrystal has small size, high crystal purity, extremely few impurity phases, low cost of raw materials, wide sources of raw materials, lower synthesis temperature and simple preparation process, and is favorable for industrialized production.