The invention discloses a preparation method of an aluminum-silicon carbide composite material. The preparation method comprises the following steps: evenly mixing coarse silicon carbide particles and fine silicon carbide particles, adding an aluminium dihydrogen phosphate water solution accounting for 1 to 3% of the total weight of two silicon carbide particles and water accounting for 5% of the total weight of two silicon carbide particles, heating, carrying out wet kneading until the humidity reaches 10% to obtain a mixture, wherein the weight percentage of the aluminium dihydrogen phosphate water solution is 50%; drying the mixture, granulating, carrying out aging to obtain powder; filling the powder into a mould, performing moulding under a pressure of 10 MPa to form a nearly moulded biscuit; encapsulating the mould containing the biscuit, sintering the mould so as to convert the biscuit into a prefabricated component; encapsulating the mould containing the prefabricated component, and soaking the mould into aluminum to obtain the aluminum-silicon carbide composite material. The preparation method has the advantages that the raw materials do not contain any paraffin micro-emulsion, the amount of aluminium dihydrogen phosphate is reduced at the same time; the porosity of the biscuit is high, the volume percentage of aluminum in the composite material is large, the thermal conductivity is increased therefore; the route is simplified, the cost is reduced, and the thermal conductivity is improved.