The invention discloses a block copolymer obtained by ring-opening copolymerization of 6,7-dihydro-5H-oxepin-2-ketone (DHO) series unsaturated lactone and lactide, and a preparation method thereof. The invention induces a DHO series unsaturated monomer into a main chain of polylactic acid to obtain an unsaturated polylactic acid block copolymer with a main chain containing a carbon-carbon double bond, and can induce a branch chain with appropriate length on the main chain of the polylactic acid by utilizing the carbon-carbon double bond with reactivity, which has great significance for improving the melt strength and processability of polylactic acid material. In addition, the preparation method of the unsaturated polylactic acid block copolymer has simple operation process, low synthesistemperature, and can adjust the chain length of each component in the unsaturated polylactic acid block copolymer and the molecular weight of the copolymer by controlling the adding amount of each reactant so as to obtain the unsaturated polylactic acid block copolymers with different properties.