The present invention relates to a method for preparing a stereoblock polylactide, comprising: a step of obtaining a first reaction mixture with a monomer conversion rate of 80 to 95% by adding a catalyst to a D-lactide and growing a PDLA chain; a step of obtaining a second reaction mixture with a monomer conversion rate of 80 to 95% by adding an L-lactide to the first reaction mixture and growing a racemic PDLLA chain at the end of the PDLA chain; and a step of further adding an L-lactide to the second reaction mixture and growing a PLLA chain at the end of the PDLLA chain through a polymerization reaction. The preparation method is capable of providing a more convenient synthesis by skipping a process of removing residual monomers in the middle of the reaction, and also of preventing multi-blocking and oligomerization of polymer chains due to a mixture of chains by gradually adding polymerizable monomers while controlling the monomer conversion rate in a one-port synthesis, thereby reducing chain transfer during the polymerization.