A liquid chromatography tandem mass spectrometry (LC-MS/MS) method was established to synchronous detect six triazine herbicide residues in seawater, including atrazine, ametryn, simazine, prometryn, cyanazine and dimethametryn. The triazine pesticides in seawater were firstly extracted with acetonitrile, and the extraction solution were purified with SPE small column made with new nano materials. Then LC-MS/MS and external standard method were used to analyze and measure these six triazine pesticides. The results showed that the average recovery rates of these six herbicides were in the ranged of 65.40%–97.40% when the concentrations of these six triazine pesticides were 0.20 and 1.00 and 20.00 μg/L, respectively, with the relative standard deviation of 0.89%–5.86% and the detection limit (sensitivity) of 0.03–0.05 μg/L. This synchronous detection method could be applied for the determination of these six triazine herbicides in seawater, with the rapid, convenient, accurate and reliable advantages. [Chinese Fishery Quality and Standards, 2023, 13(3): 26-34]