Abstract: LC-MS/MS coupled with stir bar sorptive extraction (SBSE) was applied to the determination of 4 organophosphorus pesticides residues, including malathion, triazophos, quinalphos and phosalone, in Weishan Lake water. The optimum extraction condition was achieved by optimizing the factors of SBSE efficiency, such as extraction coating, acidity of the sample solution, stirring speed, extraction time, ionic strength of the solution, desorption solution, desorption time and desorption mode. After the sample was extracted by SBSE and desorption, the analytes were separated on a ZORBAX Eclipse Plus C18 chromatographic column. The MS analysis was carried out in the multi reaction monitoring mode and external standard method was used for quantification. Linear relationships were found between the peak areas and the mass concentrations of the 4 organophosphorus pesticides in definited ranges with the detection limits (3S/N) in the range of 0.16-0.73 μg·kg-1. Recovery rates obtained by standard addition method were in the range of 72.3%-107% and RSDs (n=6) were in the range of 7.8%-11%.