Hypochlorous acid (HOCl) as the main component in chlorination and also as the innate immune factor relevant to immune defense has attracted considerable attention. Electrophilic addition reaction of olefins with HOCl, one of the most important prototype of chemical reactions, has been intensively studied for a long time; however, it has not been fully understood yet. In this study, addition reaction mechanisms and transformation products of model olefins with HOCl were systematically investigated by the density functional theory method. The results indicate that the traditionally believed stepwise mechanism with a chloronium-ion intermediate is only suitable for olefins substituted with electrondonating groups (EDGs) and weak electron-withdrawing groups (EWGs) but it is a carbon-cation intermediate that is favorable for EDGs featuring p−π or π−π conjugation with the C�C moiety. Moreover, olefins substituted with moderate or/and strong EWGs prefer the concerted and nucleophilic addition mechanisms, respectively. Epoxide and truncated aldehyde as the main transformation products can be generated from chlorohydrin through a series of reactions involving hypochlorite; however, their generation is kinetically not as feasible as the formation of chlorohydrin. The reactivity of three chlorinating agents (HOCl, Cl 2 O, and Cl 2 ) and the case study of chlorination and degradation of cinnamic acid were also explored. Additionally, APT charge on the double-bond moiety in olefin and energy gap (ΔE) between the highest occupied molecular orbital (HOMO) energy of olefin and the lowest unoccupied molecular orbital (LUMO) energy of HOCl were found to be good parameters to distinguish the regioselectivity of chlorohydrin and reactivity of olefin, respectively. The findings of this work are helpful in further understanding the chlorination reactions of unsaturated compounds and identifying complicated transformation products.