Abstract: In order to apply cypermethrin scientifically and safely to greenhouse vegetables, the residue dynamics and final residues of 10% cypermethrin EC on pakchoi in open field and greenhouse were compared by using the methods of gas chromatography and field experiment. The pakchoi samples were extracted by acetonitrile with ultrasound assistance, cleaned up by self- made florisil solid- phase extraction column and determined by gas chromatograph with electronic capture detector (GC- ECD). The mean recoveries of cypermethrin in samples ranged from 92.9% to 106.0% at the added levels of 0.1- 1.0 mg/kg with relative standard deviation (RSD) of 2.8%-8.4%, and the minimum detectable concentration was 0.01 mg/kg. The field experiments indicated that cypermethrin was rapidly degraded in pakchoi. The degradation velocity was related to cultivation methods. With double recommended dosage (effective constituent concentration was 90 g/hm2), the half-lives of cypermethrin in open field and greenhouse were 1.856 d and 2.314 d, and degraded above 90% after 7 d and 10 d, respectively. When pakchoi was treated for 3 times at an interval of 7 d with effective constituent concentration of 90 g/hm2, the safety interval of cypermethrin in pakchoi under open field was 3 days, which should be extended to 7 days under greenhouse conditions.