The acidity of chloroaluminate ionic liquids and the interaction mechanism of Lewis acid site Al2Cl7(-) of chloroaluminate ionic liquid and pyridine were experimentally investigated by IR characterization by using pyridine as molecular probe and increasing pyridine adsorption, and theoretically confirmed by quantum chemical calculations at density functional theory (DFT) and ab initio levels. It was found that the anions, Al2Cl7(-) and AlCl4(-), which could withdraw lone pair electrons of pyridine, were characteristic of Lewis acid. Therefore, they displayed pyridine coordinated to Lewis acidic site using pyridine as probe. The acidity of Al2Cl7(-) was found stronger than that of AlCl4(-) by analyzing IR absorption frequency, bond length and charge distribution. The mechanism of forming and evolvement of the Lewis acid site Al2Cl7(-) of chloroaluminate ionic liquid was proposed. When the amount of pyridine is small, only the adsorption state of Py-Al2Cl7(-) exists. The highly Lewis acidic adsorption state of Py-Al2Cl7(-) complex was converted into Py-AlCl4(-) complex and Py-AlCl3 complex with increasing pyridine contents, leading to the changes in IR absorption spectra.