A method for the determination of emamectin benzoate by ultra-performance liquid chromatography coupled with tandem mass spectrometry（UPLC-MS/MS） was developed.The analytes were extracted using ethyl acetate,and cleaned with PCX column.The column eluents were extracted by ethyl acetate again.The extracts were blown to nearly dry by nitrogen,then dissolved in methanol.The sample solutions were separated on a BEH C18 column（50 mm×2.1 mm,i.d.1.7 μm） using a gradient elution program of aqueous solution containing acetate and ammonium acetate.Qualitative and quantitative detections of the analytes were carried out under the multiple reaction monitoring（MRM） mode by MS with electrospray in positive ionization.The method had a good linear correlation（r≥0.999） in the range of 1-200 μg/L.The recoveries of emamectin benzoate at spiked level of 0.1-50.0 μg/kg were in the range of 70%-110% with relative standard deviations（RSDs） of 0.3%-15.9%.The limit of quantization（LOQ） was 0.1 μg/kg.There were significant matrix suppression effects in soil samples and rice straw samples,but matrix enhancement effect occured in water samples.