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