A method for preparing flexible capsule film by using a chitosan-sodium alginate copolymer system comprises the steps such as preparing chitosan, preparing the copolymer system, and forming film with the copolymer system. The method for preparing flexible capsule film by using the chitosan-sodium alginate copolymer system according to the invention has the advantages that utilization rate of materials is high, the production cost is low, the method is green and environmentally friendly, the production efficiency is high, the yield is up to 95.9%, the method is simple to perform, the product has good film-forming property, the film has excellent flexibility and elastoplasticity and has certain flexion elasticity and good extensional plasticity, a capsule shell prepared with the film has good stability and is free of deformation cracking or wrinkles after it is natural aired at room temperature for 24 hours, and the whole capsule shell has uniform thickness and is suitable for market popularization and application.