Abstract: Dissolved organic matter (DOM) was separated from a large volume of surface sweater in offshore Qingdao by DAX-8 and XAD-4 resins used in tandem. Four fractions, namely hydrophobic acid (HOA), hydrophilic acid (HIA), hydrophobic neutral (HON), and hydrophilic neutral (HIN) were obtained. A total of 0.3 g of marine DOM was isolated from 200 L seawater with the total dissolved organic carbon (DOC) extraction recovery of 48%. The four DOM fractions were characterized in molecular level by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).More than 4,000 individual molecules were assigned from HOA and HIA fractions, which have relatively greater O/C ratios and greater degrees of oxidative degradation.More than 1,000 individual formulas were assigned from the neutral fractions which have greater H/C ratios and lower degree of condensation. The nitrogen-containing compounds were relatively enriched in hydrophilic fractions (HIA and HIN). The resins-in-tandem method is capable for seawaters with high salinity and low DOM concentration, and can be used to isolated and fractionate DOM fractions with different polarities from a large volume of seawater. The amount and low ash content of samples can meet the requirements of characterization and biological culture experiments.