All inorganic CsPbBr3 nanocrystals (NCs) have gained significant attention due to their potential as light-emitting diodes, solar cells and other optoelectronic devices. However, to a certain extent, the stability issues of CsPbBr3 NCs under water, light, and thermal conditions limit their further applications. In order to improve stability, this article proposes a strategy of modifying CsPbBr3 NCs with poly(ethylene adipate) (PEA). The surface passivation of macromolecular polymer PEA not only increases the distance between NCs which inhibits the aggregation of small NCs, but also provides a dense protective layer to protect NCs from corrosion by water, air and light. Therefore, the performance of PEA-modified CsPbBr3 NCs in terms of optical properties and stability is significantly improved. The photoluminescence intensity of PEA-modified CsPbBr3 NCs remains at about 40% of the initial intensity when heated to 100 °C, while that of pure CsPbBr3 NCs has decreased to less than 10%. The photoluminescence intensity of PEA-modified CsPbBr3 NCs decreased to 83% of the initial intensity after 7 cycles of heating to 100 °C and then cooling to room temperature, while that of pure CsPbBr3 NCs decreased to 13%. Meanwhile, the PL intensity of PEA-modified CsPbBr3 NCs remained at 66% of the initial intensity after 10 h under 365 nm ultraviolet light, and there is no obvious red shift in PL spectra, while the PL intensity of pure CsPbBr3 NCs only maintained 21% of the initial intensity, accompanied by obvious red shift in PL spectra. The results show that the PEA-modified CsPbBr3 NCs have a positive effect on stability.