Abstract: Difenoconazole, a commonly used fungicide for the control of anthracnose in tea plantations, has not been thoroughly studied regarding its residue degradation behavior and impact on tea quality during the post-fermentation of Pu-erh tea. In this study, a three-dimensional numerically controlled box fermentation system was employed. Using sun-dried raw tea as the substrate, three concentrations of difenoconazole (low: 0.5 mg/kg, medium: 5.0 mg/kg, and high: 20.0 mg/kg) were applied to investigate its degradation kinetics and effects on the transformation of characteristic polyphenolic compounds during fermentation. The results indicated that difenoconazole degradation during Pu-erh tea fermentation followed a first-order kinetic model, with a half-life of 9.7-13.9 d. After 18 days of fermentation, the degradation rate reached 55%-68%, with a processing factor (PF) of 0.32-0.45. The conversion efficiency to the metabolite with relatively high toxic risk (CGA-205375) was low (< 2.8%). Difenoconazole residues exhibited an inhibitory effect on the degradation and transformation of tea polyphenols during fermentation, showing an overall non-linear dose-effect relationship. Within the application range of 0.5-20.0 mg/kg, the degradation extents of tea polyphenols in all difenoconazole treatment groups at the fermentation endpoint were lower than those in the blank control group, with significant intergroup differences (p < 0.05). No significant differences were observed among the difenoconazole treatment groups, indicating no notable concentration dependence. Furthermore, difenoconazole significantly inhibited the conversion of theabrubins to theabrownins, with the inhibitory effect positively correlated with concentration. After 18 days of fermentation, the reduction in thearubigins (33.0%) and the accumulation of theabrownins (11.3%) in the high-dose group were both significantly lower than those in the control group (41.2% and 13.3%, respectively). However, no statistically significant differences were observed in the transformation of theaflavins among the treatment groups. This study provides scientific data support for understanding the degradation of pesticide residues and their effects on fermentation quality during Pu-erh tea processing.