Disclosed herein is a temperature- and pH-sensitive block copolymer having excellent gel strength, including: (a) a poly(ethylene glycol)-based compound (A) or a copolymer (B) of the poly(ethylene glycol)-based compound (A) and a biodegradable polymer; and (b) a polyurethane-based oligomer (C). Further, a method of preparing a temperature and pH-sensitive block copolymer having excellent gel strength, including: (a) synthesizing a poly(ethylene glycol)-based compound (A) or a copolymer (B) of the poly(ethylene glycol)-based compound (A) and a biodegradable polymer; and (b) coupling a polyurethane-based oligomer (C) with the copolymer (B), is disclosed. Also disclosed is a polymer hydrogel type drug delivery system, including: (a) the block copolymer; and (b) a physiologically active substance which can be included in the block copolymer.