In this work, a novel binary mixture system containing the ionic liquid (IL) 1-cyanopropyl-3-methylimidazolium tetrafluoroborate ([PCNmim][BF4]) and acetonitrile (AN) was prepared. The density, dynamic viscosity, and electrical conductivity of the pure IL and the binary system at different ratios were determined at atmospheric pressure from 288.15 to 323.15 K. We have estimated a series of important thermodynamic parameters of the pure IL by empirical equations and correlated the temperature dependences of the electrical conductivity and dynamic viscosity of the binary system using both the Vogel-Fulcher-Tamman and Arrhenius equations. We also focused on the excess molar volume, V-E, of the binary mixture system, and the results of the V-E were correlated to the Redlich-Kister polynomial equation. Then, the interstice model and excess volume were employed to investigate the structural properties and packing structure, respectively, of the pure [PCNmim][BF4] and IL + AN mixtures. For a deep understanding of the intermolecular interactions of the binary system, the structure, energy, and COSMO charge distribution of the cation and anion of the IL and the AN molecule were calculated by density functional theory calculations to evaluate the intermolecular interactions of the IL and AN by means of the variations in the infrared (IR) spectrum.