To evaluate the safety of cyromazine used on celery, dynamic test and final residual test of 75% cyromazine wettable powder on celery were carried out in Lhasa of Tibet from 2019 to 2021. The ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis method was used to analyze the residues of cyromazine and its metabolite melamine in celery. The degradation rule and half-life were clarified, and the safety evaluation of the residual amount of cyromazine and its metabolite melamine were carried out. The results showed that the degradation of cyromazine on celery complied with the first-order reaction kinetics equation, with a half-life of 3.18-3.40 d, which was an easy-to-degrade pesticide. The residual amount of melamine on celery decreased first and then increased. After 2-3 days of cyromazine application, the residual amount of melamine reached the maximum, and then showed a gradual decrease trend over time. The preliminary regression analysis showed that the residual degradation dynamics of melamine was in line with the first-order kinetic relationship after its residual amount reached the maximum in celery, with a half-life of 5.13-6.24 d. According to GB 2763-2021, the maximum residue limit (MRL) value of cyromazine on celery is 4 mg/kg. Under the condition that 75% cyromazine wettable powder is applied twice or three times on celery according to the recommended dose (168.75 g a.i./hm²) and seven days after the last application, cyromazine and its metabolite melamine have no significant risk of exceeding the standard. It is suggested that the safe harvest interval for cyromazine applied to celery should be 7 days．