The invention provides an online automatic 
monitoring system of 
total nitrogen and 
total phosphorus in 
seawater and a monitoring method thereof. The 
system comprises an automatic 
seawater liquid feedcontroller, an 
ultraviolet microwave digestor, a 
total nitrogen measuring flow passage, a 
total phosphorus measuring flow passage and a spectrophotometer, wherein the automatic 
seawater liquid feed controller is connected with the 
ultraviolet microwave digestor by a liquid feed pipeline; digested sample flows into an electronic cooler from a liquor drainage pipeline of the 
ultraviolet microwave digestor to be cooled and respectively enters into the 
total nitrogen measuring flow passage and the 
total phosphorus measuring flow passage; the total 
nitrogen measuring flow passage comprises a 
nitrate ion reducing device, a sample reaction pipeline is connected with the spectrophotometer, and the front end of the sample reaction pipeline is provided with a 
nitrite color 
reagent injecting valve; and the total 
phosphorus measuring flow passage comprises the structure that the sample reaction pipeline is connected with the spectrophotometer, and the front end of the sample reaction pipeline is provided with an orthophosphate color 
reagent injecting valve. The invention can complete work procedures such as automatically sampling seawater, digesting, reducing, washing, measuring, and the like,at one time.