The chemical synthesis of proteins can help prepare proteins that are difficult to obtain using recombinant techniques, such as proteins carrying complex post-translational modifications, proteins containing multiple unnatural amino acids, proteins with non-linear structures, and mirror image proteins, expanding the ability to create new substances. We have developed a method system for protein chemical synthesis based on protein and peptide hydrazides, including peptide hydrazide solid phase synthesis, protein hydrazide recombinant expression, multi-segment hydrazide convergent ligation, protein backbone modification, protein glycosylation-assisted folding, etc. Using the method system we have achieved the chemical synthesis of more than 100 proteins such as polyubiquitin, modified histones, cytokines, GPCR, and nucleic acid polymerases, and the use of chemically synthesized proteins has generated more and more applications in biochemical mechanisms and biomedical research. As a representative case, our team investigates the use of protein chemical synthesis to resolve the biochemical and structural mechanisms of protein ubiquitination-related enzyme complexes, especially pathology-related ubiquitin E3 ligases and deubiquitinating enzymes.