The invention relates to a method for preparing a graphene polymer composite material through an in-situ polymerization method. The method comprises the following steps: adding multilayered graphene and a polymer monomer into a high-pressure kettle and injecting CO2 to enable the CO2 to reach a supercritical state; after uniformly stirring, rapidly releasing pressure of the high-pressure kettle and cooling to enable the polymer monomer to enter graphene sheet layers; adding an initiator; after sufficiently reacting, cooling to stop polymerization; adding a methanol solution and filtering a sample; drying the sample in a drying box in vacuum until the weight is constant, so as to prepare the graphene polymer composite material. According to the method provided by the invention, agglomeration of the graphene is inhibited better and the dispersion is more uniform, so that the effect of enhancing the performance of a polymer is better; an obtained product can be regulated and controlled bycontrolling polymerization reaction conditions; the multilayered graphene is peeled by adopting supercritical CO2; preparation conditions are moderate and the prepared graphene has high purity; the yield is relatively high and the CO2 has no toxicity, is economical and is easily separated from the product; the method is an environment-friendly preparation method and is low in cost and simple in treatment steps.