Asymmetric arylation of secondary phosphine oxides (SPOs) is a potential strategy to obtain P-chiral compounds, and has developed rapidly since the first report in 2016. However, the mechanism is still ambiguous and few DFT studies have been published so far in this field. Taking palladium/Xiao-Phos-catalyzed asymmetric arylation of SPOs as model, our DFT mechanism studies allows the updating of the catalytic cycle. Tautomerization of SPOs occurs first to initiate the downstream arylation transformations. DFT computations show that Pd/X1-catalyzed H₂O-assisted tautomerization is more favored, and the resulting intermediate with trivalent SPO fragment enters the downstream arylation directly. Meanwhile, reductive elimination is both the rate- and enantioselectivity-determining step in the whole catalytic cycle. Bonding and IGMH analysis on the TSs of reductive elimination indicate that, the difference in non-bonding interactions between X1 and substrates mainly determines the enantioselectivity. This work not only provides deeper insights into the self-adaptive property of SadPhos ligands but also offers valuable guidance for future related studies of SPOs.