Many rivers, lakes and reservoirs of China have occurred algal bloom events in recent years. The main reason is that the contents of total nitrogen and total phosphorus in water exceed the standard values. An automatic analysis system for detection of total nitrogen in water was developed based on a copper-clusters modified nitrate sensor and sequential injection analysis technique. The experimental results demonstrate that the automatic analysis system features a high sensitivity of 6.5 μA/(mg?L-1) and a good linearity of 0.993 with the concentration of total nitrogen ranging from 0 mg/L to 2 mg/L. Compared with other automatic instruments for total nitrogen detection, the developed system has the advantages of low reagent consumption, simple to use, low power consumption and cost.