The invention discloses a preparation method of polyacrylonitrile/graphene composite-based carbon fiber, which comprises the following steps: firstly preparing a polyacrylonitrile mixed solution uniformly dispersed with graphene through an in-situ polymerization method, and then employing the mixed solution as a spinning solution and obtaining a polyacrylonitrile/graphene composite protofilament by a wet spinning or dry-jet wet spinning process, and finally making the protofilament subject to a pre-oxidation treatment and a carbonization treatment to obtain the polyacrylonitrile/graphene composite-based carbon fiber. Compared with the existing polyacrylonitrile-based carbon fiber, the carbon fiber prepared by the method of the invention has a significantly increased mechanical property, and the carbonation yield in the preparation process is improved, therefore, the preparation method is an efficient and reliable preparation method with good application prospects.