Wet chemistry methods, including hot-injection and precipitation methods, have emerged as major synthetic routes for high-quality perovskite nanocrystals in backlit display and scintillation applications. However, low chemical yield hinders their upscale pro-duction for practical use. Meanwhile, the labile nature of halide-based perovskite poses a major challenge for long-term storage of perovskite nanocrystals. Herein, we report a green synthesis at room temperature for gram-scale production of CsPbBr3 nanosheets with minimum use of solvent, saving over 95％ of the solvent for the unity mass nanocrys-tal production. The perovskite colloid exhibits record stability upon long-term storage for up to 8 months, preserving a photoluminescence quantum yield of 63％ in solid state. Importantly, the colloidal nanosheets show self-assembly behavior upon slow solidification, generat-ing a crack-free thin film in a large area. The uniform film was then demonstrated as an efficient scintillation screen for X-ray imaging. Our findings bring a scalable tool for synthesis of high-quality perovskite nanocrystals, which may inspire the industrial optoelectronic application of large-area perovskite film.