A density functional theory study has been conducted to gain insight into the intriguing experimental observations on the synthesis of quinazoline-2,4-(1H,3H)-diones from 2-aminobenzonitriles reacting with CO2 catalyzed by protic ionic liquids (ILs). We explored the molecular mechanism of the titled reaction, as well as the origin and catalytic nature of different ILs toward the reaction in detail. The calculated energetically viable mechanism involves CO2 attack, intramolecular rearrangement, and intramolecular cyclization stages. This mechanism features the initial polarization of the C N triple bond with the assistance of the real catalytic species, [HDBU+][TFECOO-], where the cation [HDBU+] acts as Bronsted acid and the anion [TFECOO-], the adduct of anion [TFE-] and CO2, acts as a nucleophile. The calculated results present the electrostatically controlled character of the reaction, where the reactivity relies on the electrostatic interaction of the IL cation with the anion. The reactivity can be controlled and regulated by the basicity of the deprotonated counterpart of the IL cation as well as the CO2 adsorption ability of the IL anion. The best catalytic performance of [HDBU+][TFE-] is attributed to its strongest basicity of the deprotonated counterpart of [HDBU+] and its most efficient CO2 adsorption property of [TFE-]. These theoretical results are expected to provide guidance for designing efficient IL-based catalysts in preparing quinazoline-2,4-(1H,3H)-diones by reacting 2-aminobenzonitriles with CO2.