The invention provides a composite core for a power grid transmission line wire and a preparation method thereof. The composite core has high strength, high toughness, high glass-transition temperature and surface cleanness. Materials of the composite core comprise a resin material and a strengthening material. The resin material comprises a thermosetting resin, a curing agent, an accelerating agent, a releasing agent, a flexibilizer and nanometer particles. The strengthening material is one kind or a plurality of kinds of carbon fiber, glass fiber, basalt fiber, boron fiber, aramid fiber, silicon carbonate fiber and protein bound iodine (PBI) fiber. The content of composite core fiber Vf=50-80%, tensile strength of the composite core is not lower than 2100MPa, and glass transition temperature of the composite core is not lower than 180 DEG C. The toughness of the composite core is remarkably improved, and breakage and cracking probability caused by brittleness factors in production, transportation and wire hanging construction processes are reduced.