Abstract: To ensure the safe application of dimethoate to celery, the residue dynamics of dimethoate and omethoate in celery are studied. 40% dimethoate emulsifiable concentrate was sprayed at different dosages (600 and 900 g a.i/hm2) during different growth periods, including once in the middle growth stage, twice and three times in the harvest period, and celery samples were collected. Samples were extracted by acetonitrile and determined by UPLC-MS/MS. The results showed that 3 days after one time application of dimethoate in the harvest period, the residue of dimethoate was lower than MRL value, however, the residue of omethoate was over than MRL value. The preharvest interval of 21 days was suggested for omethoate, which could ensure that the residues of dimethoate and omethoate in celery were both lower than MRL value. During the middle growth stage with one time application, the residue of dimethoate and omethoate in celery was lower than MRL value. By twice application of dimethoate during the harvest period, the residue of dimethoate in celery was lower than MRL value at 7 days. By three times application of dimethoate during the harvest period, the residue of dimethoate in celery at low dose spraying was lower than MRL value at 7 days and the residue of dimethoate in celery at high dose spraying was over MRL value at 7 days. For both twice and three times application, the residue of omethoate was over MRL value at 7 days. Repeated spraying of dimethoate can cause excessive omethoate residue, therefore, the application of dimethoate during the harvest period should be prohibited.