Abstract: The rapid and accurate determination of marine dissolved inorganic carbon (DIC) is crucial for the study of marine carbonate systems. Since membrane-introduction mass spectrometer (MIMS) can efficiently measure dissolved gases, it has been successfully applied to the determination of various dissolved gases in seawater. This feature also offers a new technical approach for DIC analysis. In this study, a method for the determination of seawater DIC was developed based on a self-assembled MIMS, and the determination conditions for seawater DIC were optimized. Meanwhile, MIMS and non-dispersive infrared methods were synchronously used to measure DIC concentrations in seawater samples collected from the South China Sea. The results indicate that when the sample flow rate is controlled at 0.85 mL∙min⁻¹, the injection time is 3 min, the sample temperature is 20 ℃, and the acidification condition is the addition of 20 μL of 30% H₂SO₄, The precision of seawater DIC measurement by MIMS is satisfactory (the coefficient of variation can reach 0.2%); the results are in good agreement with those obtained by the non-dispersive infrared method (regression slope of 1.018, R²=0.994), which can meet the determination requirements for seawater DIC samples. This study develops an efficient MIMS-based method for seawater DIC determination. The method possesses promising prospects for laboratory popularization and application and can provide new technical support for the application of membrane-introduction mass spectrometry in the research field of seawater carbon cycle.