Monodisperse monomolecular methoxy PEG azide can maintain the good solubility of polyethylene glycol in water, and when applied to PEG-modified drugs, this product can improve its hydrodynamic volume while maintaining the advantages of low polymerization and weak immunogenicity of PEG. Meanwhile, this product can be used as an ADC linker for the development of antibody-coupled drugs (ADCs).