Using three pipe clamp solenoid valves to replace the traditional six-port valve for sample quota, a set of multi-channel flow injection analyzer was designed in the present paper. The authors optimized optimum instrumental testing condition, and realized determination and analysis of total dissolved nitrogen in seawaters. The construction of apparatus is simple and it has the potential to be used for analysis of total dissolved nitrogen. The sample throughput of total dissolved nitrogen was 27 samples per hour. The linear range of total dissolved nitrogen was 50.0～1 000.0 μgN·L-3 (r≥0.999). The detection limit was 7.6 μgN·L-3. The recovery of total dissolved nitrogen was 87.3%～107.2%. The relative standard deviation for total dissolved nitrogen was 1.35%～6.32% (n=6). After the t-test analysis, it does not have the significance difference between this method and national standard method. It is suitable for fast analysis of total dissolved nitrogen in seawater.