This study provides a comprehensive analysis of the concepts and assessment processes of water resources in China, focusing on the characteristics of water resources and variations in water cycle fluxes. It reveals that the distribution of water resources in China is uneven, with more south and less north, and human activities have led to a decline in water resources, particularly in northern arid and semi-arid regions. Further analysis shows that duplicated measurements of surface water and groundwater significantly affect water balance calculation and water resource assessments, serving as a crucial factor guiding water resource development and utilization. The study also finds that consistency correction of runoff series is insufficient to meet the requirements of accurate water resource assessment. It is urgent to strengthen fundamental research in hydrology and hydrogeology, and to establish a dynamic assessment system for the efficient management and rational use of surface water and groundwater in the current changing environment.