An antibody-drug conjugate (ADC) of formula (I) or a pharmaceutically acceptable salt or solvate thereof is provided. In formula (I), p is an integer ranging from 1 to 26, A is an antibody, and -(L-D) is a linker-drug unit. L is a linker unit having a glycopeptide, and D is a drug unit. The antibody is conjugated to the linker unit through a cysteine residue of the antibody. A method for forming an antibody-drug conjugate (ADC) is also provided. A-(L-D)p  (I)