The thia -Diels-Alder reaction represents a versatile synthetic method for the preparation of six-membered sulfur-containing compounds. However, the mechanism of the thia -Diels-Alder reactions remains unclear. In this work, time-resolved spectroscopic experiments and DFT calculations demonstrate that phenacyl sulfide undergoes Norrish II cleavage to produce thioaldehyde, and ortho-hydroxy benzhydryl alcohol occurs in a dehydration reaction to generate o -QMs using diphenylphosphate as the catalyst. Then, the thia -Diels-Alder reaction takes place between thioaldehyde and o -QMs by an asynchronous concerted mechanism. The illustration of the thia -Diels-Alder reaction mechanism not only provides important support for organic synthesis and drug design but also enhances fundamental insights into reaction pathways and catalytic processes in the field of chemical synthesis.