The invention relates to a four-step process 3D-braided two-ply board type fiber hybrid prefabricated member and a preparation method thereof. The hybrid prefabricated member comprises two different kinds of braided yarns, and is characterized in that a fabric structure of the prefabricated member is a four-step process 3D-braided structure, two kinds of braided yarns are distributed in a two-ply board structure in the prefabricated member, and connection of two-ply boards is realized between the two kinds of knitted yarns through a hybrid braided structure. The preparation method provided by the invention is based on a four-step-process 3D braiding process and is characterized in that a 3D fiber hybrid braided structure is used as a connecting unit to form a fiber hybrid connecting region, so that the connection of two braided yarn plates at two sides of the prefabricated member is realized, yarn winding is performed in a partitioning manner, and braiding is performed in a partitioning manner. Two side plates of the two-ply board in the hybrid prefabricated member are connected together by using braiding fibers, so that the prefabricated member is high in structure integrity and stable in structure mechanical property, does not need a subsequent stitching procedure, is wide in application range, and can be suitable for various forms of braided yarns such as single yarns and plied yarns; mechanization can be realized for movement of the braided yarns in a braiding manner without human assistance, so that the preparation efficiency is improved.