1-Benzyl-3-alkyl benzimidazolium ionic liquids was synthesized by reaction of benzyl bromide with mono-alkylated benzimidazole under microwave radiation, wherein mono-alkylated benzimidazole was synthesized from benzimidazole and alkyl halide in the presence of PEG-6000 as phase change catalyst. The structure of the 1-benzyl-3-alkyl benzimidazolium ionic liquid was characterized by FT-IR, 1H NMR and13 C NMR. The 1-benzyl-3-alkyl benzimidazolium ionic liquid was used as flame retardant of gel coat resin, and the gel coat resin achieved the best flame retardancy when the dosage of the 1-benzyl-3-alkyl benzimidazolium ionic liquid was 0.2phr.