Abstract: Conventional assessment cannot fully depict the impact of river system evolution on flood risk in recent years. In this study, a four-dimensional assessment system of flood risk was proposed considering the evolution of the river network system in Xiangjiang River Basin. A comprehensive and quantitative investigation was also made on the spatiotemporal evolution of the river network. The entropy-weight method was used to verify the assessment system of flood risk from 1995 to 2015 from four dimensions: the risk of disaster-causing factors, the sensitivity of disaster-bearing environment, the vulnerability of disaster bearing body, as well as the ability of disaster prevention and mitigation. The certain objectivity was achieved in the difference from the previous subjective assignment. A flood risk was then expressed by the maximum precipitation, the number of days with daily rainfall greater than or equal to 50 mm, the maximum continuous precipitation, and the longest continuous precipitation days over 10 years. The sensitivity of the disaster environment was represented by a composite index of the absolute elevation, relative elevation, vegetation cover, and water system evolution. The vulnerability of disaster-bearing bodies was indicated by the population per unit area, GDP per unit area, and the percentage of arable and construction land. The disaster prevention and hazard reduction capability were explained by the number of beds, the ratio of medical service personnel, the capacity of reservoirs per unit area, and the financial income per capita. The results show that: 1) The river network system presented high spatial heterogeneity in the rapid descending trend over the past 20 years. The river tended to be shrunk and simplified. The drainage density and the tributary development coefficient indicated the highest attenuation rate up to 30.96 % and 34.01 %, respectively. 2) The high-risk region was located in the eastern and southern areas. The sensitivity of the disaster environment showed a pattern of high in the north, but low in the southeastern and southwestern. The vulnerability of disaster-bearing bodies was mainly distributed in economically developed areas. The areas with high disaster prevention and mitigation capacity were mainly located in the municipal districts with high economic levels. Among them, Zixing City shared stronger water retention and water connotation capacity. The local flood risk gradually increased to expand the range of high-risk flood hazards in the basin over the past 20 years. High-risk areas mainly included the southern Yongzhou City, the northwestern Chenzhou City of the upstream, the southeastern of Hengyang City, and the municipal districts of Changsha City. 3) Rapid urbanization led to the decline of the river network system and storage capacity but also resulted in the increase of extreme precipitation and flood frequency over the past 20 years. Therefore, a four-dimensional assessment system was established for the flood risk considering the evolution of the quantitative water system, indicating the impact of water system evolution on the flood risk under the background of urbanization in a more scientific and delicate way. The finding can also provide scientific support for agricultural production in the basin, particularly for regional flood control and disaster reduction.