Objective To establish a method for determination of pyriclobenzuron (PBU) residues in rice and paddy environments, and to determine the residual amounts and observe the degradation dynamics of PBU. Methods In July 2022, the paddies of Zhejiang Academy of Agricultural Sciences were selected as experimental fields, and were divided into the blank control group (no pesticide application), the 1-fold-concentration pesticide group (1 kg/667 m²), and the 5-fold-concentration pesticide group (5 kg/667 m²), with a 100 m² area in each group. At the early tillering stage of rice, 20% suspension of PBU sulfate was sprayed once in the 1-fold-concentration and 5-fold-concentration pesticide groups, and rice plants, paddy water and soil samples were collected 2 h, and 1, 2, 3, 5, 7, 11, 14, 21, 28, 35, 49 d and 63 d following spraying PBU, while rice straw, field soil, brown rice and rice husk samples were collected 98 d following spraying. PBU was extracted and purified in samples using a quick, easy, cheap, effective, rugged, and safe (QuEChERS) pretreatment technique, and the PBU contents were determined in samples using ultrahigh performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). The solvent standard working solution and matrix standard working solution were prepared. A linear regression equation was fitted between PBU concentration (x-axis) and peak area (y-axis), and the ratio of the slope (k) of the matrix standard curve to the slope (K) of the solvent standard curve was calculated to evaluate the matrix effect of PBU in samples. According to the Guidelines for Pesticide Residue Testing in Crops (NY/T 788—2018), the addition levels of PBU were set at 0.005, 0.050, 5.000, 1 000.000 mg/kg in rice plants, 0.005, 0.050, 2.000, 10.000 mg/kg in paddy water, 0.005, 0.050, 2.000 mg/kg in soil, and 0.005, 0.050, 5.000 mg/kg in brown rice and rice husks. The recovery and relative standard deviation (RSD) of PBU addition were calculated to evaluate the effectiveness of UPLC-MS/MS for determination of PBU contents. The first-order kinetic equation of PBU concentration was fitted in samples at different sampling time points to analyze the trends in PBU degradation in rice plants, paddy water, and soil, and the half-life of PBU was calculated in different samples. Results There was a good linear relationship between the mass concentration and peak area of PBU at concentrations of 0.000 1 to 0.020 0 mg/kg under solvent and matrix conditions (R² = 0.985 8 to 0.999 7, t = -0.47 to 1.62, all P values < 0.01). The matrix effects of PBU were 70.26%, 65.42% and 65.12% in rice plants, brown rice and rice husks, indicating a matrix-inhibitory effect, and the matrix effect was 87.06% in soils, indicating a weak matrix effect. The recovery of PBU addition was 77.61% to 100.12% in different samples, with RSD of 1.43% to 6.74%, and a limit of quantification (LOQ) of 0.005 mg/kg, and the addition recovery and RSD met the requirements of the Guidelines for Pesticide Residue Testing in Crops (NY/T 788—2018), validating the effectiveness of UPLC-MS/MS assay. Following spraying PBU at a dose of 1 kg/667 m², the half-life of PBU was 6.24 d in rice plants and 3.43 d in paddy water samples, respectively. The final residues of PBU were lower than the LOQ of 0.005 mg/kg in brown rice and rice husk samples 98 d following spraying PBU. Following spraying PBU at a dose of 5 kg/667 m², the half-life of PBU was 15.75 d in rice plants and 7.62 d in paddy water samples, respectively. The final residue of PBU was lower than the LOQ of 0.005 mg/kg in brown rice 98 d following spraying PBU, and the final residue of PBU was 0.049 mg/kg in rice husks. Conclusions A simple, and highly accurate and precise UPLC-MS/MS assay has been developed for determination of PBU residues in rice plants and paddy environments through extraction and purification of PBU from matrix samples using QuEChERS pretreatment. After spraying PBU in paddies, the concentration of PBU gradually decreases in rice plants and paddy water over time, and the final residual concentration is low.