Disclosed is a depolymerized glycosaminoglycan from Thelenota ananas(dTHG), weight average molecular weight of which is about 8000 ~ 20000Da, and monosaccharide components of which are acetylgalactosamine(GalNAc), glucuronic acid(GlcUA), fucose(Fuc) or their sulfates(expressed as -OSO), in which molar ratio of GalNAc: GlcUA: Fuc: -OSO is about 1: (1±0.3 ): (1±0.3): (3.5±0.5). Said dTHG is a potent endogenous factor X enzyme inhibitor, which has good anticoagulating and antithrombotic activity, and can be used to prevent and/or treat thrombotic diseases. Also provided is a method for preparing said dTHG, which comprises steps of 1) extracting and obtaining fucosylated glycosaminoglycan(THG) from Thelenota ananas body wall； 2) depolymerizing THG to obtain dTHG by method of peroxide depolymerization or method of peroxide depolymerization catalyzed by catalyst of the fourth period transition metal ions； 3) removing oligomer and/or polymer impurities in dTHG.
