The invention provides a transparent gas barrier composite film and its preparation method. The transparent gas barrier composite film is formed by having polyvinyl alcohol, graphene oxide and a crosslinking agent undergo blending, crosslinking, film casting and isothermal crystallization processes where a weight ratio of graphene oxide to polyvinyl alcohol is 0.1 wt %, graphene oxide induces crystallization of polyvinyl alcohol to form a hybrid structure compose of polyvinyl alcohol crystals, graphene oxide and the crosslinking agent, crystallinity of polyvinyl alcohol is more than 30%, transmittance of the composite film is more than 85%, and the oxygen transmission rate of the composite film is less than 0.005 cc/m2/day.