A block copolymer and a method for preparing the same are disclosed. The method comprises the following steps: (A) mixing a compound of formula (I), a catalyst of formula (II), and a first solvent to obtain a first mixture; (B) adding a first monomer into the first mixture for reaction to obtain a second mixture; and (C) adding a second monomer into the second mixture for reaction to obtain a third mixture; wherein the compound of formula (I) and the catalyst of formula (II) are as defined in the specification.