The invention discloses a barrier heat sealing type biaxial tensile composite thin film and a preparation method thereof. The thin film comprises two surface layers, and at least a barrier layer, wherein the surface layer and the barrier layer are connected through bonding layers. The thin film is characterized in that the two surface layers comprise the following components, by weight, 90-99% of metallocene linear low density polyethylene resin with a characteristic of bimodal molecular weight distribution, 0-5% of an antistatic agent, 0-5% of an anti-adhering agent, 0-5% of a smoothing agent, and 0-5% of an antioxidant; the barrier layer is a barrier polymer, and the bonding layer is a high strength and high temperature resistance adhesive. The preparation method adopts a multi-layer co-extruding successive biaxial tensile process or a multi-layer co-extruding simultaneous biaxial tensile process. The composite thin film has excellent gas and moisture barrier property, and the mechanical properties of the thin film are further improved. The thin film has characteristics of high tensile strength, low temperature high heat sealing strength, and high puncture resistance strength.