Based　on　4-methylthiazole,　a　two-step　method　was　used　to　synthesize　four　sulfonic　acid　functionalized　ionic　liquids　with　different　anions　to　catalyze　the　transesterification　reaction　of　soapberry　oil　with　methanol　to　produce　biodiesel.　The　characterization　results　of　Fourier　transform　infrared　spectroscopy,　nuclear　magnetic　resonance,　and　thermogravimetric　analysis　suggested　that　ionic　liquids　are　successfully　prepared　with　high　thermal　stability.　Among　the　ionic　liquids　investigated,　3-(3-sulfonic)　propyl-4-methylthiazole　trifluoromethane　sulfonate　([Ps-MTH][CF3SO3])　performs　the　highest　catalytic　activity.　In　particular,　catalyzed　by　[Ps-MTH][CF3SO3],　the　optimum　operating　conditions　of　transesterification　of　soapberry　oil　and　methanol　are　128　(temperature),　28.10:1　(molar　ratio　of　alcohol　to　oil),　0.62　mmol/g　(catalyst　amount,　based　on　the　mass　of　oil)　and　8　h　(reaction　time),　leading　to　a　high　biodiesel　yield　of　92.78%±0.47%.　Besides,　[Ps-MTH][CF3SO3]　also　possesses　good　reusability　and　universality.　This　study　will　provide　basic　data　for　the　industrial　production　of　biodiesel　from　soapberry　oil　catalyzed　by　ionic　liquids.　©　2021,　Chemical　Industry　Press　Co.,　Ltd.　All　right　reserved.