Abstract: U-Pb age compositions of fluvial detrital zircon comprehensively reflect tectonic, magmatic, and metamorphic activities of zircon sources. It serves as a reliable tracer for provenance analysis and source-to-sink processes, and has become popular in revealing river evolution and quantifying the relative contribution of sediments. However, a few studies on modern fluvial detrital zircon have raised concerns that zircon age distributions are affected by sediment transport, sorting, and mixing processes. Therefore, it is crucial to examine the controlling and influencing factors of zircon age distributions in fluvial source-sink systems. We studied the U-Pb ages of detrital zircon in seven catchments in Zhejiang and Fujian provinces, from which the distribution characteristics of detrital zircon U-Pb ages were determined using laser ablation-inductively coupled plasma (LA-ICP-MS) technology. Through bulk samples and zircon grain-size analyses, the potential influence of hydrodynamic sorting on the age spectrum of the basins was revealed. Results show significant spatial differences in the U-Pb age distributions of detrital zircon in the rivers in Zhejiang and Fujian Provinces. The detrital zircon U-Pb ages of the Qiantang River are characterized by three main age groups: 80~180 Ma, 190~524 Ma, and 677~916 Ma. The Jiaojiang River has only one age peak of the Yanshanian period at approximately 125 Ma. There are two characteristic age groups, 113~190 Ma and 1707~1897 Ma, in the detrital zircon of the Oujiang River. The detrital zircon of the Minjiang River has four main age intervals: 76~244 Ma, 374~457 Ma, 732~816 Ma, and 1622~1935 Ma. The main zircon age peaks of the Mulanxi River detrital zircon are the Cretaceous peak, ca. 100 Ma, and the Jurassic peak, ca. 142 Ma. The ages of detrital zircons in the Jinjiang River Basin are all concentrated between 78 and 175 Ma. The detrital zircons in the Jiulong River have three age groups: 75~120 Ma, 120~180 Ma, and 192~313 Ma. Nevertheless, the age spectra of detrital zircons in the seven rivers all contain groups from the Jurassic-Cretaceous of the Yanshanian period, ranging from 120 to 180 Ma. The spatial distribution characteristics of detrital zircon age in the Zhejiang-Fujian provinces are associated with the large-scale magmatic activities in East China, which were caused by the subduction of the Pacific Plate beneath the Eurasian Plate since the Mesozoic. More interestingly, zircon morphology analysis revealed no significant correlation between the zircon grain size of the seven rivers and their U-Pb ages, indicating that provenance, rather than hydrodynamic sorting, had a significant impact on the zircon age spectrum. However, study on zircon equivalent sphere diameter and morphology showed a trend that the older the zircon, the higher the roundness, which suggests that the distribution of zircon grain shapes is influenced by the long-term transportation and sedimentary cycling processes.