The residue levels of fluazinam in root mustard were investigated by using a quick, easy, cheap, effective, rugged, and safe (QuEChERS) technique with ultra-performance liquid chromatography tandem mass spectrometry. Samples of leaf and root mustard were analyzed. The recoveries of fluazinam were 85.2–110.8% for leaf mustard with the coefficient of variation of 1.0–7.2%, and 88.8–93.3% for root mustard with the coefficient of variation of 1.9–12.4%. The suspension concentrate formulation of fluazinam was applied on root mustard at 262.5 g a.i. ha −1 in accordance with good agricultural practice (GAP), respectively. After the final application, the root mustard samples were collected at 3, 7, and 14 days. Fluazinam residues in root mustard were less than 0.01–0.493 mg kg −1 . The dietary risk of fluazinam was predicted by comparing intake amounts with the toxicological data, namely acceptable daily intake (ADI) and acute reference dose (ARfD). The risk quotient (RQ) was 72.2–74.3%, for ordinary consumers, which showed negligible risk. According to the maximum residue limit (MRL) and dietary risk assessment, it is suggested that the pre-harvest interval (PHI) of 3 days; meanwhile, the MRL of 2 mg kg −1 was suggested for fluazinam in root mustard, which indicates that the dietary risk of fluazinam 500 g L −1 suspension concentrate (SC) with the recommended usage on root mustard is negligible. This study provided basic data on the use and safety of fluazinam in root mustard to help the Chinese government formulate a maximum residue level for fluazinam in root mustard.