In urban landscape rivers, algal bloom caused by excess nutrients broke out regularly in summer due to the acceptance of wastewater from different sources. The characteristics of DOM in the river water were changed because of the broken out of the algae bloom, and thus the migration and transformation of pollutants, which affected by DOM in the river water, was affected as well. In this study, the water samples before and after the algal blooms in the main stream of the Haihe River were collected to study the influence of outbreak of algae blooms on DOM in water bodies in urban landscape river. The results showed that the content of DOM in the river water increased from 26.47 to 38.20 mg·L-1. The contents of TN, NH+4-N and TON in the river water were 3.1 times, 2.5 times and 4.2 times higher than those before the algal bloom broke out, respectively, while, the contents of TP and NO-3- remained stable. C/N ratio decreased from 18.51 to 6.39 while N/P ratio increased from 5.69 to 20.10. The source of DOM changed from terrestrial source mainly to aquatic source mainly and the water also showed a gradual trend to phosphorus limit. Three-dimensional fluorescence spectra showed that the composition of DOM changed from the source of complex organic matter to the source of algae organic matter. The results of UV absorbance indicated that the molecular structure of DOM was changed to be more complex. However, the content of humus-liked in water decreased while the content of protein-liked increased. Ultrafiltration experiments were also conducted to study the molecular weight of DOM before and after algae blooms. It was found that DOM with small molecular weight had the dominates, in which DOM with molecular weight less than 10 kD accounted for more than 80% of the total DOM. With the increase of molecular weight, the intensity of all peaks of three-dimensional fluorescence spectra showed an increasing trend and the UV absorbance also indicated that the high molecular weight DOM might contain more aromatic compounds. The results suggested that the role of DOM should be paid attention to the eutrophication in urban landscape river water. Especially when external pollutants are controlled, the contribution of DOM degradation to eutrophication should be fully considered.