Abstract: A simple, fast and easy operation method was developed for determining thiamethoxam residues in spinach by liquid chromatography-mass spectrometer(LC-MS). The final residue and decline trend of thiamethoxam in greenhouse and open field were studied. The results showed that the decline dynamic of thiamethoxam in spinach in two different cultivation methods accorded with the first-order kinetics, and the decline rate of thiamethoxam in greenhouse spinach was slower than the other. The half-lives of thiamethoxam were 4.08 d and 1.28 d, respectively. When the spinach in greenhouse and open field was sprayed 2-3 times at an interval of 7 d with thiamethoxam 250WDG with recommended dosage of 30 g a.i/hm and 1.5 times recommended dosage 45 g a.i/hm, 3, 5, 7 days after the last application, the final residues of thiamethoxam in greenhouse spinach were higher than thiamethoxam residues in open field spinach at harvesting. There is no MRL of thiamethoxam spinach set in China now. Referring to the MRL of 0.2 mg/kg in cabbage, the safe intervals of thiamethoxam in greenhouse spinach and in open field spinach should be more than 7 d and 3 d, respectively. From this study, in order to ensure the dietary risk in acceptable level, different pre-harvest intervals of thiamethoxam in spinach must be set according to different cultivation methods.