Provided are a BAB-type tri-block copolymer including a polylactic acid (A) that is a hydrophobic block and polyethylene glycol (B) that is a hydrophilic block, a method of preparing the BAB-type tri-block copolymer including synthesizing a polylactic acid (A)-polyethylene glycol (B) di-block copolymer by a ring opening polymerization of a lactic acid monomer using a hydroxyl group of polyethylene glycol (B) as a polymerization initiator; and synthesizing a BAB-type tri-block copolymer by reacting the AB-type di-block copolymer with polyethylene glycol (B) including at least one carboxyl group at one or more terminals thereof in the presence of a coupling agent and a catalyst, and a drug delivery system using the BAB-type tri-block copolymer.