The invention relates to a class of novel Brønsted acidic ionic liquids each having a lactam cation, and to a method for preparing the same through simple neutralization reaction of a lactam, which is available in large scale from industry, with a Brønsted inorganic or organic acid under room temperature. The properties of the lactam Brønsted acidic ionic liquids are as follows: they are water- and moisture-stable; they are more environmentally benign and lower cost than that of dialkylimidazolium salts; they have stronger Brønsted acidity and can be used as acidic catalysts and media instead of inorganic corrosive acids, such as concentrated sulfuric acid and hydrofluoric acid, in many acid-catalyzed reactions; and they can also be used as green media for extraction and separation.