The invention discloses composite modified emulsified asphalt and a preparation method thereof. The composite modified emulsified asphalt is prepared from the following raw materials: matrix asphalt, an SBS (Styrene Butadiene Styrene) modifier, polyphosphoric acid, glycerol triglycidyl ether, a composite emulsifier, a stabilizer and water. The preparation method comprises the following steps: heating matrix asphalt, adding glycerol triglycidyl ether, conducting stirring, and conducting shearing to obtain a mixture A; adding an SBS modifier into the mixture A, uniformly conducting stirring, conducting swelling, carrying out low-speed shearing, and carrying out high-speed shearing to obtain a mixture B; adding polyphosphoric acid into the mixture B, conducting stirring, and conducting developing in a drying oven to prepare composite modified asphalt for emulsification; adding a stabilizer and a compound emulsifier into water, uniformly conducting mixing, and regulating the pH value to 2-3 by using industrial hydrochloric acid to prepare liquid soap; heating the liquid soap and keeping the temperature; and adding the composite modified asphalt into the soap liquid, and conducting emulsifying to obtain the composite modified emulsified asphalt. The composite modified emulsified asphalt has the advantages that the solid content is high, the storage stability is good, the elastic recovery capability is relatively strong, and the high-temperature and low-temperature performances are excellent.