The present invention relates to a method for preparing a Prussian blue coating film for an electrochromic device, more particularly to a method for preparing a Prussian blue coating film for an electrochromic device comprising an electrochromic layer coated on a light transmitting substrate including a coating of a transparent electrode, an ion storage layer coated on another light transmitting substrate including a coating of a transparent electrode and an electrolyte layer formed between the electrochromic layer and the ion storage layer, wherein the ion storage layer is formed with a specific nano-dispersed composition comprising Prussian blue by wet coating, thereby providing comparable or better physical properties, including light transmittance, response time, durability, etc., when compared with the conventional electrochemical techniques, simplifying the manufacturing processes and significantly improving the productivity. With improved economic advantages, the present invention enables the realization of large-sized electrochromic devices.