UV and fluorescence spectra were applied to analyze the characteristics of reaction of disinfectants (free chlorine, free bromine) with tryptophan, as well as the formation of disinfection byproduct-trihalomethanes (THMs). The results show that disinfecting reaction decrease both the intensity and FRI (fluorescence region integration) of tryptophan. A linear relationship was found between the two indexes. Increasing the ratio of free bromine led to a lowering of FRI, a decrease in UV280 and an increase in UV254. Bromine-substitution ratio of THMs also went up with increasing the ratio of free bromine, whereas the total THMs concentration didn’t follow such mono-increase pattern. Those phenomena are mainly due to a higher substitution but a lower oxidation capacity of free bromine, when compared with free chlorine. A linear relationship between FRI and CHBr3 concentration was found using free bromine alone, which did not exist by dosing ammonia to change the characteristic of disinfectant.