[Objective] The temporal and spatial distributions of outbound water quality of the Jiangxi-Guangdong boundary in the source region of Dongjiang River were analyzed in order to provide a theoretical basis and data support for the water environment management and water ecology improvement in this watershed. [Method] Based on the 11 water quality indicators of the monitoring section during 2007—2019, the methods of descriptive statistical analysis, water pollution index method, correlation analysis and Mann-Kendall test and other methods were used to to analyze the changes and causes of outbound water quality. [Results] ① Water quality was different in different hydrological periods, especially in the spatial and temporal variation of chloride, sulfate and ammonia nitrogen. ② The water quality in the flood season was better than that in the non-flood season, and the inferior V water only existed in the non-flood season. During the inter-annual period, the outbound water quality was the worst in 2008, and it started to be improved in 2009, and during 2017—2019, they tall met the requirements of class Ⅱ—Ⅲ. ③Ammonia nitrogen was the most important pollutant of outbound water quality, but its concentration had decreased dramatically. Additionally, rare earth mining, fruit industry development and large-scale breeding were the main factors affecting water quality. [Conclusion] During 2007—2019, the water quality of the outbound water in the source area of the Dongjiang River was slightly different in different hydrological periods, but the overall trend was downward. The concentration of the main pollutant ammonia nitrogen dropped significantly. The protection and restoration measures adopted in the source area in recent years have achieved remarkable effects.