In this paper, we established a sequential injection method to monitoring the concentration of total dissolved nitrogen (TDN) in seawater, which combined with an UV-thermal digestion. The sequential injection method consumes less reagents and less time, mixes more evenly than the flow injection method. TDN detection process mainly includes three parts: the digestion process of conversion organic nitrogen to nitrate, the reduction process of nitrate to nitrite, and the process of reaction between nitrite and chromogenic agent. The UV-thermal digestion that transforms dissolved organic nitrogen (DON) into nitrate has the highest digestion efficiency. The nitrate reduction process with a UV lamp is considered a greener choice which avoids the toxic substance and complicated operation of cadmium column reduction. We analyzed the optimal concentrations of all the reagents used in the experiment with the univariate experimental design. The reaction conditions were optimized by using three kinds of DON, which were C2H5NO2, CH4N2S and C10H14N2Na2O8. Under the optimal reaction conditions, we tested the seawater in the Qingdao Sculpture Garden and the recovery rate of adding standard. In May, the TDN concentration of seawater in Qingdao Sculpture Garden was 122.94μg/L, the relative standard deviation was 0.34%. The recovery rate was as high as 99.94% to 100.39%. The outcomes indicate that the proposed method can apply to the potential in-situ monitor.