Two-dimensional transition metal dichalcogenides (TMD), such as molybdenum disulfide (MoS2), have aroused substantial research interest in recent years, motivating the quest for new synthetic strategies. Recently, halide salts have been reported to promote the chemical vapor deposition (CVD) growth of a wide range of TMD. Nevertheless, the underlying promoting mechanisms and reactions are largely unknown. Here, we employ first-principles calculations and ab initio molecular dynamics (AIMD) simulations in order to investigate the detailed molecular mechanisms during the salt-assisted CVD growth of MoS2 monolayers. The sulfurization of molybdenum oxyhalides MoO2X2 (X = F, Cl, Br, and I)─the form of Mo-feedstock dominating in salt-assisted synthesis─has been explored and displays much lower activation barriers than that of molybdenum oxide present during conventional "saltless" growth of MoS2. Furthermore, the rate-limiting barriers appear to depend linearly on the electronegativity of the halogen element, with oxyiodide having the lowest barrier. Our study reveals the promoting mechanisms of halides and allows growth parameter optimization to achieve even faster growth of MoS2 monolayers in the CVD synthesis.