The emergence of green materials has attracted considerable attention in the field of optoelectronics. Copper-based lead-free halide perovskite (with a near-unity quantum yield) obtained from Cs3Cu2I5 nanocrystals (NCs) can exhibit blue emission with a wavelength of 440 nm and provide outstanding stability for various applications. However, in practical applications, colloidal dispersion purity and film quality are inadequate toward a high-performance device. In this study, antisolvent-free gel permeation chromatography is used to purify Cs3Cu2I5 NCs. The purified Cs3Cu2I5 NCs exhibit a high photoluminescent quantum yield and provide a highly oriented single-crystal film. Density functional theory calculation results indicate that the iodide-rich phase in the NCs makes them highly stable. In addition, it has been demonstrated for the first time that the mixture of polymethyl methacrylate (PMMA) and Cs3Cu2I5 NCs has waterproofing capabilities. The composite film consisting of Cs3Cu2I5 NCs and PMMA can survive in water for several days. This result opens up more possibilities for the application of these green material.