New experimental data for solubility of carbon dioxide (CO2) in the aqueous solution of 1-aminopropyl-3-methylimidazolium bromide ([APMIm]Br) with four different water mass fractions (0.559, 0.645, 0.765 and 0.858) at T= (278.15-348.15) K with an interval of T= 10 K and p = (0.1237-6.9143) MPa were presented. The electrolyte nonrandom two-liquid (eNRTL) model was modified to be applicable for an ionic liquid (IL) aqueous solution system, by introducing an idle factor beta to illustrate the association effect of IL molecules. A solution Henry's constant for CO2 solubility in the IL aqueous solution was defined by introducing a correlative coefficient R-s(*). The vapor-liquid phase equilibrium of the [APMIm]Br-H2O-CO2 ternary system was successfully calculated with the modified eNRTL model. The chemical and physical mechanisms for the ionized CO2 formation and the molecular CO2 dissolved in the solution were identified. The effects of aqueous medium on both chemical and physical dissolution of CO2 in the [APMIm]Br aqueous solution were studied, and a considerable enhancement of the solubility of CO2 with increase of the water content in the solution was observed. (c) 2016 Elsevier Ltd.