A home-made automatic analyzer to determine the dissolved organic carbon (DOC) in seawater was introduced in this paper. This analyzer was based on ultraviolet-potassium persulphate oxidation method. The analyzer has 7 parts, including sampling, purging of inorganic carbon, enter sample, oxidation, cooling, apart of gas/liquid and analysis of CO2. The program was compiled by programming language of Visual Basic as the control system. All of the action of the analyzer is auto controlled. Different samples are separated by worked air and Milli-Q water and enter the system using six-way valve. With the catalysis of ultraviolet radiation, the organic matter in samples can be oxidated to CO2 quantitatively by K2SO2O8 and analyzed by non-dispersive infrared (NDIR) detector latter. The results showed that the oxidation efficiency, as well as the detect limit and the veracity are satisfying. The home-made analyzer was compared with the high temperature oxidation method, and the results showed that the two methods have little difference. The cost of the instrument is cheaper, and need not expensive catalyst, so the DOC in seawater can be analyzed with much lower prices.