The acidity of chloroaluminate ionic liquids (ILs) was determined by using pyridine and acetonitrile as IR spectroscopic probes. Based on the characterization method of acidity of solid acid, IR vCCN absorption frequencies of pyridine in ionic liquids were assigned. By using the pyridine probe, it was found that when the anion molar composition x of ionic liquid varies within 0.4-0.5, ionic liquids exhibit weak Lewis acidity. Strongly basic molecular pyridine can be used as a probe to measure the acidity of ionic liquids whether their acidity is strong or weak, while weakly basic molecular acetonitrile is only fit for strong acid. In addition, the Lewis acidity-activity correlation for the chloroaluminate ionic liquids catalyst in the alkylation reaction was studied. The weakly acidic ILs with x < or = 0.5 does not exhibit any catalytic activity in the alkylation reaction. When the anion molar composition x is more than 0.55, the activity of ionic liquids is greatly enhanced due to the increase in the strength of the strong Lewis acidic species Al2Cl7-. But with the increase in alkenes conversion, the selectivity of 2-alkylbenzene is slightly reduced.