The invention discloses a high-foaming polystyrene-methacrylic acid-naethylmethacrylate gasified expendable casting material and a preparation method thereof. A functional methacrylic acid monomer is added in the preparation process of the styrene-naethylmethacrylate expendable casting material. The process is as follows: deionized water, a styrene monomer, a methacrylic acid monomer, the naethylmethacrylate monomer, a dispersant and an initiator are mixed, subjected to a suspension polymerization reaction, and prepared into a particle material; the prepared particle material is washed, dried, sieved and stored to obtain a pre-foaming material; the pre-foaming material, a foaming agent, a plasticizer, a surface coating agent and an antistat are placed in a high-pressure kettle to foam and is subjected to temperature keeping, cooling, discharging and washing, as a result, the high-foaming expendable casting material is obtained. According to the high-foaming expendable casting material and the preparation method thereof, the volume of a material in unit mass is greatly increased, the economical benefit of the material is improved, the generation of a cracked gas is reduced, the improvement of an workshop environment is facilitated, and meanwhile pollution to the environment is lowered.