Selective CF bond activation and functionalization of readily available polyfluorinated groups is a classical and effective way to access high value-added organofluorine compounds. Insights into origin of the selectivities could provide valuable guidance to precise manipulation and so as to get desired products. In this study, the mechanism and causation of the selective mono-CF bond allylation of trifluoromethylalkenes with tetrabutylammonium fluoride (TBAF) as catalyst has been investigated by using the density functional theory (DFT) methods. It is unveiled that the S_N2′-type nucleophilic substitution going through the addition-elimination mechanism is more favorable than the concerted S_N2 pathway. According to the calculation results, allyl anions would kinetically identify the trifluoromethylalkene, ammonium cation (TBA⁺), and difluoromethylalkene in turn, which guarantees the mono-CF bond activation under the conditions of excessive nucleophiles. Furthermore, as a specific point, the regioselectivity of the reaction using asymmetric allyl anion as the nucleophile has also been studied. The Parr function calculations and non-covalent interaction analysis reveals that the more significant nucleophilicility of the benzyl carbon atom in allyl anion and the π^…π interaction between the aryl substituents on the two reacting fragments leads to favor of the reaction at the benzyl site.