The invention discloses a preparation method of stable lead-free all-inorganic double perovskite A2BB'X6 nanocrystals. The method adopts a thermal injection technology to synthesize the nanocrystals,and concretely comprises the following steps: mixing a metal precursor salt, a reaction solvent oleic acid, oleylamine, octadecene and other raw materials, carrying out vacuum heating and stirring ata certain temperature for a certain period of time, rising the temperature to a suitable reaction temperature under the protection of N2, rapidly injecting the obtained hot oleate solution of A into areaction system, and then rapidly cooling the system to room temperature by using ice bath to finally obtain the A2BB'X6 perovskite nanocrystals having a uniform size. The method has the advantages of simplicity, convenient, good reappearance and environmental protection, and the obtained product has the advantages of uniform size, good dispersion, high stability and excellent photocatalytic performance, and can be applied to the fields of photocatalysis, photodetectors, laser, solar cells and the like.