A composition is prepared from a platinum-based antitumor compound and a novel compound, and the novel compound is sodium(S)-2-(dithiocarboxylate ((2S, 3R, 4R, 5R)-2,3,4,5,6-pentahydroxyhexyl)amino)-4-(methylthio)butyrate. The novel compound in the composition could effectively chelate the platinum-based antitumor compound and reduce the various adverse effects exhibited by the platinum-based compound. The dosage of the platinum-based antitumor compound can thus be increased, and the antitumor effect can be optimized. The novel compound itself is low-toxic and it has low chelating activities with essential metals in the human body; it would not impose significant adverse effects on the human body at given doses. The composition and its application as an antitumor drug overcome some technical drawbacks of platinum-based antitumor drugs, namely severe adverse effects and the development of drug resistance during administration.