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BACKGROUND: Thiamethoxam is widely used to control pests in Chinese kale, popularly consumed leafy vegetables. The potential risk to the environment and human health has aroused much public concern. Therefore, it is important to investigate the degradation behavior, residue distribution and dietary risk assessment of thiamethoxam in Chinese kale. RESULTS: A sensitive analytical method for determination of thiamethoxam and its metabolite clothianidin residue in Chinese kale was established and validated through a quick, easy, cheap, effective, rugged, and safe (QuEChERS) technique with ultra‐performance liquid chromatography tandem mass spectrometry (UPLC–MS/MS). The recoveries were 85.4–101.2% for thiamethoxam and 79.5–108.1% for clothianidin, with the relative standard deviations (RSDs) of 0.9–10.2% and 1.8–6.0%, respectively. For the dissipation kinetics, the data showed that thiamethoxam in Chinese kale was degraded with the half‐lives of 4.1 to 4.5 days. In the terminal residue experiments, the residues of thiamethoxam were 0.017–0.357 mg kg−1 after application 2–3 times with a preharvest interval (PHI) of 7 days under the designed dosages. The chronic and acute dietary exposure assessment risk quotient (RQ) values of thiamethoxam in Chinese kale for different Chinese consumers were 0.08–0.19% and 0.05–0.12%, respectively, and those of clothianidin were 0.01–0.04% and 0.02–0.04%, respectively, all of the RQ values were lower than 100%. CONCLUSION: Thiamethoxam in Chinese kale was rapidly degraded following first‐order kinetics models. The dietary risk of thiamethoxam and clothianidin through Chinese kale was negligible to consumers. The results from this study are important reference for Chinese governments to developing criteria for the safe and rational use of thiamethoxam, setting maximum residue levels (MRLs), monitoring the quality safety of agricultural products and protecting consumer health. © 2021 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]