Abstract: In recent years, methylation reactions have garnered increasing attention in both organic and biosynthesis fields. By introducing methyl groups at appropriate positions in compound structures, the molecular chemical structure, physicochemical properties, and biological activity can be altered, imparting new functionalities. This facilitates the creation of more purpose-specific functional molecules. The enzymatic-catalyzed alkylation reactions in biocatalysis, known for their high chemical, regional, or stereo selectivity, have gained widespread attention and played a crucial role in the biosynthesis of natural products.The introduction of methyl groups through methyltransferases (MT) offers a novel avenue for precisely designing the structure and function of target molecules, thereby modulating their biological activity and physicochemical properties. Notably, halide methyltransferases (HMT), a unique subclass of MT, not only catalyze the generation of diverse halogenated hydrocarbons but also achieve enzymatic, in-situ regeneration of the expensive cofactor S-adenosylmethionine (SAM) in the presence of cost-effective, non-natural methyl donors like iodomethane. This review highlights the latest research advancements and breakthroughs in HMT, showcasing the development of a simple and versatile SAM recycling system through the introduction of HMT-MT cascade, thereby enhancing the atomic economy of reactions. The identification of enzymes within the thiopurine methyltransferase family (TPMT) addresses environmental concerns related to methyl donors. Furthermore, variants obtained through directed evolution optimize HMT, facilitating the transfer of longer chain alkyl groups. These innovative studies provide novel strategies for efficient biological alkylations, potentially revolutionizing green biomanufacturing technologies.