The mechanism is investigated for regioselective [3 + 3] cycloaddition of vinyl sulfoxonnium ylide with cyclopropenone in switchable mode. For reaction catalyzed by NHC-Cu, the coordination with vinyl sulfoxonnium ylide produces copper carbene after releasing of Cl and DMSO. The annulation is realized via bonding sequence of first C1-C6 then C3-C4 leading to 2,3,4,5-tetrasubstituted phenol as major product. The outcome turns to be another tetrasubstituted phenol conformer under metal-free condition with a bonding sequence of first C3-C6 then C1-C4. Both of the two cases are featured by vinyl sulfoxonnium ylide as C3-synthon not C2-synthon involving ring transformation. Based on comparison between two possible paths, the regioselectivity of dominant product is kinetically controlled. The positive solvation effect was suggested by decreased relative and activation energies in dioxane and TFE solution. These results are supported by Frontier MO analysis on specific TSs, ELF isosurface, DI and MBO value of vital bonding, breaking.