The invention relates to a preparation method of a small-size high-stability perovskite nanocrystalline Cs4PbBr6, and belongs to the technical field of preparation of semiconductor nanomaterials. Thepreparation method comprises the following steps: firstly injecting a caesium carboxylate solution into a lead bromide solution at the temperature of 140 DEG C to 200 DEG C under the protection of N2,reacting, synthesizing a perovskite nano-block CsPbBr3 with the size of 3.8 nm to 11.8 nm, purifying the perovskite nano-block CsPbBr3, dispersing the perovskite nano-block CsPbBr3 by using octadecene, raising the temperature to 25 DEG C to 120 DEG C, injecting an ammonium bromide solution, reacting for 3-8 minutes, then centrifuging and purifying to obtain a nanoparticle Cs4PbBr6. The preparation method has the characteristics of easy operation, simple steps and rapid reaction, the raw materials are easy to get, the prepared particles Cs4PbBr6 are uniform in size, regular in morphology and excellent in stability, and the particle keeps unchanged even after a stock solution is placed in the air for a month.