Abstract: In this paper, total organic carbon analyzer coupled with stable isotope mass spectrometer（TOC-IRMS）technology is used to study carbon isotope composition and dissolved organic carbon (DOC) in water using high temperature oxidation method. According to the composition characteristics of DOC, five water soluble compounds, namely caffeine, glucose, phthalic acid, potassium acetate and sodium humate, are selected from the aspects of oxidation in order of difficulty and molecular structure to prepare DOC solutions. By changing the experimental parameters, such as solution carbon concentration, oxidation tube temperature, oxygen admission time and carrier gas flow rate, the effect of different experimental conditions on the carbon conversion rate and isotope fractionation of five compounds are studied. The parameters of TOC-IRMS are set for oxidation temperature at 850 ℃, the oxygen supply time at 20 s (flow rate at 10 cm³/min), and the carrier gas flow rate at 80 cm³/min. The measured average carbon conversion rate of DOC solutions of five compounds in different concentrations is between 95.69%~103.57%, the δ¹³C are basically consistent with the calibration reference values, and the difference range is between －0.82‰~0.55‰. Under the experimental conditions, the relative standard deviation of DOC content in different types of water samples is less than 3.7%, the standard deviation of δ¹³C value is less than 0.2‰. Results show that the data obtained by TOC-IRMS online high-temperature oxidation method met the accuracy requirements.