The invention discloses a preparation method for stable full-inorganic perovskite A2PdX6 nanocrystalline with no lead and low band gap (A=Cs, Rb, K or multiple mixing; X=Cl, Br, I or multiple mixing thereof), the preparation method takes A2PdX6 micron-crystalline as a precursor material, and then the nanocrystalline is synthesized by using a rapid antisolvent method, the main synthesis process isas follows: Firstly, A2PdX6 micron-crystalline is synthesized by a solution method or a solid phase method; then the obtained A2PdX6 micron-crystalline is dissolved in a strong polar solvent under heating condition to obtain a clear and transparent precursor solution; after the precursor solution is cooled to room temperature, a certain amount of the precursor solution is rapidly added to the antisolvent under the stirring condition, and A2PdX6 nanocrystalline colloidal solution with a particle size about 3 nm is rapidly formed, the synthesis method provided by the invention is fast, simple, can be operated at normal temperature, has good reproducibility and is environmentally friendly; at the same time, the product has uniform size, good dispersibility, high stability and excellent photoelectric response; the preparation method can be used in fields such as light or photoelectrocatalysis, photodetectors, solar cells.