An isotope dilution/gas chromatography-inductively coupled plasma mass spectrometric（ID/GC-ICP-MS）method was developed for the separation and determination of trace of methylmercury（MeHg）in seawater after an automatic distillation.Results showed that the distillation pretreatment for the seawater samples could be completed in 45 min using an automatic distillation apparatus，and BrCl solution was very efficient in removing the MeHg residues in the distillation ware，which could result in the procedural blank lowering markedly.The artificial MeHg during the distillation process might be negligible if the total mercury mass concentration in the sample was lower than 1.0 μg·L-1.The spiked recoveries for MeHg in seawater ranged from 98.2% to 122%，with RSDs for MeHg in natural seawater and spiked seawater of 16% and 5.4%，respectively，and the detection limits were 0.009 ng·L-1.The method was applied in the analysis of MeHg in the surface seawater collected from Dalian Bay with average MeHg concentration of （0.103±0.036） ng·L-1，and it could meet the requirements for analysis of trace methylmercury in coastal water.