Enzyme-protein conjugates, or cross-linked enzymes, a highly sensitive and specific molecular probe is prepared by connecting it to an antibody or antigen protein molecule in the form of covalent bond and ionic bond. It is widely used in analytical research and technical determination in the fields of medical pathology, immunohistochemistry, molecular biology, biopharmaceutical and veterinary medicine. A typical example is the binding of “activated” horseradish peroxidase (HRP) to target antibodies, proteins in immunoassay. On the other hand, as a carrier-free immobilization technology, the cross-linking of enzyme molecules can expand the industrial enzyme pool by increasing the stability during use and storage. Coupling reactions with proteins or peptides are usually carried out by small molecular connectors (such as glutaraldehyde) or enzymes (such as transglutaminase), also known as cross-linking.