The invention relates to a method for preparing a collagen film, relating to a food packaging film. The method comprises the following steps of: immersing the raw materials, such as fish skin, fish scales, fish bones or fish muscles and the like, by using an alkaline solution, after rinsing the raw materials to be neutral by using clear water, immersing the raw materials by using a hydrochloric acid aqueous solution, rinsing the raw materials to be neutral by using clear water again, then extracting in hot water, and mixing after adding an edible plasticizer into the obtained collagen aqueoussolution so as to obtain a collagen film solution; after heating and dissolving an aqueous solution containing an edible hydrophobic substance and an edible emulsifier, carrying out homogenization and emulsification treatment by utilizing a tissue pounding machine, and preparing a hydrophobic substance emulsion; and, after mixing the modulated collagen film solution and the hydrophobic substance emulsion, carrying out homogenization and emulsification treatment, after defoaming, uniformly coating the mixture on organic silicon resin, after drying, cooling the organic silicon resin to room temperature, and stripping the film so as to obtain the collagen film.