A new ionic liquid catalyst [BzMim][BF4] was synthesized through a two-step reaction. First, [BzMim]Cl was synthesized by 1-methylimidazole and benzyl chloride, and then exchanged with NaBF4 to prepare [BzMim][BF4]. Its catalytic performance and product selectivity were investigated in the Blanc chloromethylation reaction of toluene. And the experimental results indicated that [BzMim][BF4] could catalyze the chloromethylation reaction using hydrochloric acid as reactant. Compared with similar reactions catalyzed by other ionic liquid catalysts in literature, the yield of reaction catalyzed by [BzMim][BF4] was similar with that using dry HCl catalyzed by ionic liquid C12mimBr; and the selectivity of mono chloromethylation was obviously higher than the reported 35% in the case of hydrochloric acid catalyzed by ionic liquid [emim]BF4. And when the mole ratio of [BzMim][BF4] to toluene was 0.3, the mole ratio of PF to toluene was 1.3, the mole ratio of hydrochloric acid to toluene was 7.5, and when the reaction was carried out at 80 oC for 8 h, the yield of the target product methyl benzyl chloride was as high as 88.5%, and the selectivity of mono chloromethylation was more than 99%.