Disclosed is an ammonium salt with a structural formula represented by formula (I). A synthesis method thereof comprises using 8 mol% zinc chloride as a catalyst, 5 mmol of 1-phenyl-1,3-butanedione, 1.2 g of benzylamine, and 40 mL of chlorobenzene as a solvent, refluxing for 2 days, and obtaining a crystalline compound of ammonium salt precipitate. The use of the ammonium salt crystal I as a catalyst exhibits excellent catalytic performance in the cyanosilylation reaction of benzaldehyde and the reaction between benzaldehyde and benzamide. The conversion rates thereof are respectively 39.7% and 62%.