The invention relates to a composite modifier, prepared through mixed loading of aluminum-antimony-copper-ferrosilicon alloy and nanometer magnesium oxide-boron nitride, for gray pig iron and a preparation method of the composite modifier. The modifier is prepared from the following raw materials in parts by weight: 1-2 parts of nanometer magnesium oxide, 2-3 parts of nanometer boron nitride, 1-3 parts of aluminum powder, 0.8-1 parts of antimony powder, 3-5 parts of expansible graphite powder, 3-4 parts of copper powder, 4-6 parts of silicon iron powder with 45% of silicon content, 0.2-0.3 part of aluminum-magnesium silicate, 20-25 parts of ethanol water at the concentration of 5-8%, and 0.4-0.5 part of an additive. The nano-powder and the manganese-molybdenum-copper-ferrosilicon alloy powder loaded on the expansible graphite powder are mutually bonded and loaded to prepare the composite modifier with stable properties, the effect of promoting generation of crystal nucleus and refining the crystal nucleus is achieved, the composite alloyed powder optimizes the matrix structure, refines the graphite body and improves the strength, the modifier is rapidly melted after being added into the molten iron, the graphitization process is promoted, the comprehensive mechanical property of the gray pig iron is effectively improved, and compared with the traditional modifier, the composite modifier is more convenient to use, and the production cost is also reduced.