Provided are a preparation method of amino-acid ionic liquid capable of selectively reducing crotonaldehyde in mainstream cigarette smoke and an application of the amino-acid ionic liquid. The preparation method is characterized in that carboxyl in amino acid and hydroxyl in tetraalkyl ammonium hydroxide are subjected to acid-base neutralization reaction to synthesize the tetraalkyl-ammonium-hydroxide-type amino-acid ionic liquid, the prepared amino-acid ionic liquid is applied to cigarettes in a coating mode, the crotonaldehyde in the mainstream cigarette smoke can be effectively reduced, the selective reduction rate can reach 20%, and the sensory quality of the cigarettes cannot be influenced. The preparation method has the advantages that the prepared amino-acid ionic liquid is simple in synthesis method, low in preparation cost and easy to industrialize. Meanwhile, current filter-stick structures and filter-stick forming and processing technologies are not changed.