Water pollution control and prevention require that the water quality testing is of real time, online and portability. The presejt paper provides a water detecting system based on UV-visible spectra. The R2 of the linear regression is greater than 0.99 between absorbance and COD of the samples. Single measurement is within 1s. At the same time, two methods were presented for direct comparison and normalization of the spectra and their corresponding indicator parameters. By a large number of experiments with standard solution of the potassium hydrogen phthalate and water samples, we found that these methods can be used to classify samples, find suitable mathematical model of the test water from the database, to improve the universal ability of the UV water testing technology, and get other water quality indices.