Abstract: The water pollution problem has become increasingly prominent, which becomes a major factor restraining ecosystem water supply services. However, the impact of water quality and the valuation of water supply service are often neglected in current research. It is still difficult to link management practices to water quality, water supply service and its value, which limits its application in the decision-making of water resources and water environment management. Therefore, a link was established between water quality and quantity, water supply services, and water supply value based on the water resources and water quality. The quantification and valuation of water supply service, and proposed management suggestions for Xiamen City were conducted accordingly. In this study, the SWAT (Soil and Water Assessment Tool) model was employed to simulate water quantity. The market value method and the environmental restoration cost method were employed to evaluate the value of water supply service. The concepts of major excessive pollutant equivalent and water quality equivalent were proposed to estimate the water quality. The results showed that the water supply quantity of the ecosystem in 2015 was 9.26×10⁸ m³, with a value of 14.82×10⁸ RMB in Xiamen. Compared with 2010, the water supply quantity increased by 1.77×10⁸ m³ and its value increased by 2.60×10⁸ RMB. Deterioration of water quality decreased the value of water supply service by 0.31×10⁸ RMB in 2015 and 0.27×10⁸ RMB in 2010, respectively. The water supply service potential in Xiamen City was large with obvious spatiotemporal variations among different water bodies, and it was affected by overall declining water quality. Further research on the improvement of water conservation capacity and water quality should be conducted to enhance the water supply service. The method and results are understandable for non-technical decision makers and allow easy operation and comparison. While the impact of economic development and the pricing strategy of water quality are not fully considered yet, the local economic development level should be considered, and the wastewater treatment cost should be revised in future research.