The invention discloses a modified acrylonitrile-styrene copolymer and a preparation method thereof. The preparation method comprises steps as follows: a blend is obtained by mixing an acrylonitrile-styrene copolymer, filler and a processing aid; the blend is added to an extruder via a main charging port, and glass fiber is added under traction of an auxiliary charging port of the extruder; the modified acrylonitrile-styrene copolymer is generated through mixing extrusion of the extruder at a preset main machine rotating speed and extruding processing temperature. A technology used in the whole preparation process is simple, and the prepared modified acrylonitrile-styrene copolymer has excellent use performance, tensile strength, impact resistance, bending strength and size stability and can meet the severe application environment requirements.