A method for determination of 20 organic phosphorus and pyrethriod pesticides in water was established by using ultra performance liquid chromatography-tandem mass spectrometry after direct injection or after extraction and concentrated. The instrument conditions and pretreatment conditions were optimized. The regression equation of 20 kinds of pesticides showed good linear relationship, the correlation coefficients were all greater than 0.999. The detection limits were in the range from 0.21 to 2.47 μg/Lfor direct injection, and the average recoveries ranged from 81.0% to 119% with the relative standard deviations less than 10%. For extraction and concentration method, the method detection limits of the target compounds were in the range from 0.0022to0.0043μg/L, and the average recoveries ranged from 71.5% to 115% with the relative standard deviations less than 15%. This method is simple and fast, and its precision and accuracy can meet the demands of determination of pyrethriod prsticide and organic phosphorus pesticide in water.