Abstract: Residual concentrations of cyromazine and its degradation metabolite melamine in fruiting body and casing soil of Agaricus bisporus were monitored by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) and isotope-labeled internal standard method. Samples were extracted with methanol-water (4:1, V/V), purified by a strong cation exchange-solid phase column (MCX-SPE), and separated through a HILIC column. The samples were then monitored by UPLC-MS/MS using ¹³C-melamine as the isotopic internal standard. The results showed that the limit of detection (LOD) of cyromazine and melamine were both 0.001 mg/kg. Within spiked concentrations between 0.02-1.0 mg/kg, the recovery rates of cyromazine and melamine in both casing soil and fruiting body were 86%-102% (RSD ≤ 5.3%) and 98%-103% (RSD ≤ 2.4%), respectively. With spiked concentrations of 1, 10, 50, and 250 mg/kg, the dissipation of cyromazine in casing soil followed the first-order kinetics with an average half-life of 31.3 d. The cyromazine was degraded into melamine by the end of the second flush mushroom dspicked, with the conversion percent of 8.58%-13.87%. After the application of cyromazine with high concentrations (50 and 250 mg/kg), the residue of cyromazine and melaminein is higher in the first flush mushrooms than that in the second flush mushrooms. And the highest detected concentrations of cyromazine and melamine were 2.36 mg/kg and 1.75 mg/kg, respectively. To meet the requirement of environmental and food safety, the application amount of cyromazine was recommended to be no more than 50 mg/kg.