The invention relates to a preparation method of a high barrier property polyethylene/nylon 6 in-situ nanocomposite material. The preparation method comprises the following step of: carrying out reactive extrusion on polyethylene, caprolactam, montmorillonoid subjected to intercalation treatment, a compatilizer, an ion initiator and a catalyst prompter together to prepare the high barrier property polyethylene/nylon 6 in-situ nanocomposite material. The preparation method has simple process route and can realize the large-batch production of the high barrier property polyethylene/nylon 6 in-situ nanocomposite material; nano materials in the prepared nanocomposite material are evenly to disperse; and the high barrier property polyethylene/nylon 6 in-situ nanocomposite material has excellent isolation properties on air, hydrocarbon solvents and moisture and can be processed into products with higher permeability resistance property, such as thin films, vessels, bottles, and the like.