The invention provides a polystyrene composition for packing plates and a preparing method of the polystyrene composition. The preparing method mainly includes the following steps that 1, durian shells with white flesh removed are broken into pieces, washed, boiled in water, rinsed, dried and smashed, and durian shell powder is obtained; 2, the durian shell powder is subjected to heating reflux with a sulfuric acid solution and a citric acid solution in sequence, and pretreated durian shell powder is obtained; 3, the pretreated durian shell powder is subjected to activation and carbonization with potassium oxalate, and durian shell carbon powder is obtained; 4, the durian shell carbon powder is subjected to a grafting reaction with epoxy chloropropane, silicon tetrachloride and the like, and grafted durian shell carbon powder is obtained; 5, PE and other components are stirred into a stirring material, the stirring material and a foaming agent are mixed and then subjected to extrusion forming, and the polystyrene composition for packing plates is obtained. The polystyrene composition prepared through the preparing method has excellent fire resistance and biodegradability and is environmentally friendly.