Abstract: A system and method for online determination of total phosphorus in environmental water samples by phosphomolybdate blue spectrophotometry with ultraviolet direct activated potassium persulfate for digestion were designed and developed. The system parameters and method were optimized and verified, then applied to determination of the total phosphorus in the actual water sample. The water sample and potassium persulfate solution were injected into a quartz spiral tube and mixed spontaneously, and the water sample was digested by potassium persulfate which was activated by ultraviolet irradiation. The absorbance was measured at 700 nm after reduction and coloration with ascorbic acid and ammonium molybdate. Under the optimized conditions of quartz spiral tube temperature at 20℃, and the flow rates of water sample, potassium persulfate, ascorbic acid and ammonium molybdate at 1.00, 0.15, 0.06, 0.08 mL·min⁻¹, respectively, the digestion rate reached more than 97%. Linear relationship between absorbance and mass concentration of total phosphorus was found in the range of 0.1−5.0 mg·L⁻¹, with detection limit (3s/k) of 0.007 mg·L⁻¹. The results of water samples determined by this method were compared with those given by the national standard GB 11893—89, and the relative errors ranged from−4.0% to−0.50%. Test for recovery was made on the lake water samples according to the standard addition method, giving results in the range of 93.0%−100%, with RSDs (n=6) of the determined values less than 3.0%.