Amine-PEG-CH2OOH(NH2-PEG-CM) is a linear heterobifunctional PEG reagent containing an amine and a carboxylic acid. The carboxylic acid and primary amine can react to form stable amide bonds in the presence of an activator. The PEG moiety in PEG derivatives can provide water solubility, compatibility. It can be widely used in the development of antibody drug conjugates (ADCs).