Post-translational modifications (PTMs) are covalent and usually enzymatic modifications on proteins. Common PTMs include phosphorylation, acetylation, ubiquitination, glycosylation, and so on. These PTMs can cause huge influences on protein structure, distribution, and function, hence, increasing the complexity of proteome to a greater extent. Protein PTMs is generally analyzed by measuring the mass increased on the modified peptides. Since PTMs are generally present in very low abundance, specific enrichment procedures are required before PTMs identification.