The invention provides a chemical method for synthesizing flaky CuxSy nanocrystalline optoelectronic film controllably at a low temperature. The method comprises the following steps: firstly adding sulfur powder in a container, secondly adding organic solvent N,N-dimethylformamide or absolute alcohol, wherein the volume of organic solvent is more than a half of the capacity of the container; standing at 0-60 DEG C for 1h to ensure that the sulfur powder dissolved in organic solvent is saturated, horizontally placing substrate material with a new and clean metal copper surface on the bottom of the container to avoid directly contacting with sulfur powder, reacting at 0-60 DEG C for 5-24h, cleaning the product with absolute alcohol, drying at room temperature to in-situ prepare the big area film material composed of flaky CuxSy nanocrystalline on the metal copper surface of the substrate material, wherein x:y=1-2. The method of the invention adopts low temperature, has low energy consumption, is simple, green and environmentally friendly and particularly applicable to large-area industrial production, and does not use any template, add any surfactant and perform complicated post-processing operations such as purification.