Abstract: As the “China Water Tower”, Three-River-Source has a prominent water ecological status. It is of great significance to carry out the assessment of water ecosystem service function and the construction of water security pattern for maintaining national water ecological security. In this paper, the Water Yield module of the InVEST model and Nutrient Delivery Ratio module were used to analyze the spatial and temporal patterns of water production service (water quantity) and water quality purification service (water quality) in Three-River-Source from 1980 to 2020. The minimum cumulative resistance model (MCR) was used to construct the water ecological security pattern of Three-River-Source. The results showed that: (1) The average annual water yield of Three-River-Source was 600.08×108m3, showing a fluctuating increasing trend as a whole, and a spatial pattern of “high in the south and low in the north”. The areas with increased water yield depth were mainly located in the northwest, east and southeast of Three-River-Source, while the areas with decreased water yield depth were in the south and southwest edge of Three-River-Source. (2) The TN (nitrogen), TP (phosphorus) and water purification output of the Three-River-Source showed a trend of decreasing first and then increasing. The average annual output was 84628.37 t, 8167.32 t and 92795.69 t, respectively. The high output area showed the spatial characteristics of “concentrated distribution in the south and southeast, scattered distribution in the east”. (3) In terms of the construction of water security pattern, 34 water ecological source patches (area of 4626.28 km2) were identified, and 199 water ecological corridors (total length of about 9943.92 km) were extracted, which showed the spatial characteristics of “dense in the middle and east, sparse in the west, and east-west extension”.