Studying the relationship between ecosystem patterns and water conservation services is important for promoting ecosystem management and protecting the water supply. Based on the InVEST model, landscape ecology theory, and Pearson correlation coefficient analysis, this study explored the relationship between ecosystem patterns and water conservation services in the Nianchu River Basin during 2010—2020. The study yielded several interesting results: \mathrm{①} Grasslands account for more than 78% of the area, with stable bare land but active bidirectional transitions with grasslands. \mathrm{②} From 2010 to 2020, there was a slight increase in the fragmentation level of the composite ecosystem patterns within the basin. Grassland had a significantly higher Percentage of Landscape (PLAND), Mean Patch Size (MPS), and Aggregation Index (AI) than other ecosystem types. Bare land exhibited the most irregular patch shapes. Forest showed the most significant increase in Patch Density (PD) and a slight decrease in MPS. Urban areas expanded continuously. \mathrm{③} The total water conservation services for 2010, 2015, and 2020 were 272 million m³, 95 million m³, and 247 million m³, respectively, with grasslands contributing nearly 80% of the total. \mathrm{④} There is a significant correlation between the composite ecosystem pattern and water conservation services. Water conservation services are positively correlated with PD, Edge Density (ED), Landscape Shape Index (LSI), Modified Simpson’s Diversity Index (MSIDI), and Modified Simpson’s Evenness Index (MSIEI) but negatively correlated with MPS and AI. However, the correlations between different types of ecosystem patterns and water conservation services show significant variations. For example, PLAND and MPS are negatively correlated with water conservation services for grasslands but positively correlated with glaciers. In summary, grasslands dominate the ecological patterns of the Nianchu River Basin, and their degree of fragmentation is positively correlated with water conservation services. Furthermore, the relationships between different types of ecological patterns and water conservation services exhibit significant differences. This study provides a scientific basis for regional ecosystem management and water resource protection.