The invention discloses a method for preparing 2,4,5-triaryl substituted imidazole through catalysis of a degradable acidic ionic liquid, belonging to the technical field of preparation of chemical materials. According to the preparation method, a mol ratio of dibenzoyl to aromatic aldehyde to ammonium acetate is 1:1:(2-4), the molar weight of a degradable acidic ionic liquid catalyst accounts for 3-5 percent of that of the used aromatic aldehyde, and the volume (ml) of alcohol as a reaction solvent is 1-3 times molar weight (mmol) of that of the aromatic aldehyde. The method comprises the steps: performing a reflux reaction for 0.5-2 hours, rotatably evaporating a solvent after the reaction is ended, separating out a large quantity of solids after adding water, grinding solids, standing, carrying out suction filtering, washing filter residue, recrystalizing by using alcohol after drying, and drying in vacuum to obtain 2,4,5-triaryl substituted imidazole. Compared with a preparation method of other acidic ionic liquid catalysts, the method has the advantages of good activity of the catalyst, simplicity, convenience and low cost in the whole preparation process, and the like, and is easy to degrade, and brings the convenience to industrialized large-scale production.