Disclosed herein is a method for preparing a food packaging film containing chitosan comprising 90.0 to 98.0% by weight of polyethylene resin and 2.0 to 10.0% by weight of a premix containing chitosan, in which the premix comprises fine flakes of polyethylene film and chitosan powder in a ratio of 80.0 to 99.5% by weight: 0.5 to 20.0% by weight. According to the method, in order to evenly disperse and distribute chitosan in the food packaging film, chitosan powder is previously mixed with synthetic resin film flakes to prepare a premix. The premix is mixed with a synthetic resin in a predetermined ratio and extruded to prepare the food packaging film. As a result, it is possible to industrially mass-produce the food packaging film, which has chitosan powder particles evenly dispersed and distributed therein and maintain excellent elongation and tensile strength.