The invention relates to a laser-marked polystyrene composition and a preparation method thereof. The composition comprises the blend of the following components: 100 weight parts of polystyrene and 0.1-10 weight parts of powdered rubber, wherein the powdered rubber is composed of powdered styrene-butadiene rubber and powdered carboxylated styrene-butadiene rubber. The preparation method comprises the steps of proportioning the components including polystyrene and powdered rubber, melting, and blending. The composition overcomes the problems that the polystyrene cannot conduct laser marking and an excessive amount of laser auxiliary in material can cause the decrease in physical and mechanical properties and light transmittance and the increase in price. The preparation method is simple and reliable.