By inserting cysteine (C) into a heavy chain and/or a light chain of a target antibody at specific insertion site, and performing a site-specific conjugation through a free thiol group (-SH) from the site-specific inserted cysteine and a linker conjugated with a highly potent small molecule cytotoxin, a cysteine modified antibody-drug conjugate with good homogeneity is provided. The specific insertion sites of cysteine are position 205 and/or position 206 (Kabat numbering scheme) of the light chain of the antibody, and/or position 439 (Kabat numbering scheme) of the heavy chain.
