This paper presents a system for on-line determination of chemical oxygen demand (COD) and total phosphorus (TP) in water by using Fenton reagents via a micro-spectrometer. The system uses Fenton reagents with ultrasonic-assisted to realize on-line digestion of organic compounds and organic phosphorus compounds in water samples under normal temperature and normal pressure, and uses a micro-spectrometer to realize multi-wavelength spectrophotometric detection. The results show that the system realize less measuring time, lower power consumption, simpler system structure and less secondary pollution. Moreover, the results show that the relative error is ≤10%, the limit of detection (LOD) are 2 mg·L-1 COD and 0.008 mg·L-1 TP, respectively, the sensitivities are 0.021 3 COD and 0.452 6 TP, respectively, and the precision is around 5.6% RSD at 15.0 mg·L-1 COD (n=7) and 5.8% RSD at 0.010 mg·L-1 TP (n=7). In addition, there are not significantly difference between the Chinese national standard analysis methods and the methods used in proposed system.