The structure of penicillin sulfoxide is rearranged to cephalosporins by the Morin rearrangement.It is a unit reaction for the preparation of various types of cephalosporins.In order to make better use of the reaction and in view of the shortage of the reaction theory,this study used m062x/6-311++G (d,p) to explore the possible ring-opening reaction of the penicillin sulfoxide.It is found that the isomer of (S)-sulfoxide is a necessary structure.At the same time,the intramolecular hydrogen bonding effect between the side-chain amide proton (-CONH-) and the sulfinyl oxygen (-SO) is the decisive structure factor for the formation of alkenyl in ring-opening reaction,and the best reaction path is S0-＞ TS2-＞ IN1 channel.The main effect of acid catalysis is to catalyze the dehydration reaction of sulfenic acid to form sulfur cations for subsequently ring closing reaction.