▎ 摘　　要

NOVELTY - Polymer composite material comprises micro-nano hybrid structure filler, where the composite material comprises graphene nanosheets, magnesium salt whiskers and a polymer matrix. USE - Polymer composite material. ADVANTAGE - The polymer composite material is easy to carry, safe and environment-friendly, improves comprehensive mechanical property of the composite material. DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method for preparing a polymer composite material, which involves: (A) adding soluble carbonate or alkali to the aqueous solution in which the stabilizer and stirring to obtain a mixed solution A; (B) dissolving soluble magnesium salt in water to obtain a uniform solution B under stirring, adding solution B to the solution A to obtain a flocculent mixed solution C, allowing to stand for a period of time to make the insoluble magnesium salt after crystal nucleation, growth, precipitation, separation and drying, magnesium carbonate or magnesium hydroxide whiskers; (C) calcining the magnesium carbonate or magnesium hydroxide whiskers in a high-temperature furnace to remove free water, bound water and carbondioxide molecules to obtain high purity, high crystallinity and high specific surface area magnesium oxide whiskers; and (D) adding graphene nanosheets and magnesium salt whiskers containing the magnesium carbonate, magnesium hydroxide and/or magnesium oxide whiskers to the polymer matrix in a certain proportion and melting by a twin screw extruder, and mixing to obtain high thermal conductivity composite material.