a methoxypolyethylene glycol - polylactic acid block copolymer is disclosed , the methoxypolyethylene glycol - polylactic acid block copolymer being a block copolymer formed by ring opening polymerization of d , l - lactide and methoxypolyethylene glycol . a mass ratio of the methoxypolyethylene glycol to the d , l - lactide is 1 : 0 . 55 - 0 . 65 or 1 : 0 . 73 - 0 . 89 or 1 : 0 . 91 - 0 . 99 . a preparation method of the foregoing block copolymer is also provided . when the block copolymer is used as a carrier for preparing a drug micelle , the encapsulation efficiency of the drug micelle prepared after being re - dissolved by water can be greater than 90 % at 12 hours .