The effects of water and alcohol on the alcoholysis mechanisms of N-methyl-1,2-thiazetidine-1,1-dioxide and the differences of these effects have been studied using the Density Functional Theory (DFT) method at the B3LYP/6-31G* level. The results have been compared with a nonassisted study carried out previously. The results show that the water- and alcohol-assisted alcoholysis of N-methyl-1,2-thiazetidine-1,1-dioxide reduces the active energy greatly compared to the nonassisted reaction. The most favored pathway for alcoholysis of 1,2-thiazetidine-1,1-dioxides is pathway a of the stepwise reaction. The orientation of attack of CH3OH on S1 is from the more hindered side of the S1. The energy barrier of the cleavage of the C-S bond and producing N-methyl-N-ethyl-amino-methyl sulfonate (P1) is the highest among all of reaction pathways, and this may be the possible reason of having no experimental evidence on the formation of product P1. The results also show that reactions that are alcohol-assisted have a little higher energy barrier than one under water-assisted alcoholysis of N-methyl-1,2-thiazetidine-1,1-dioxide.