Studying the changes in water conservation service (WC) in Sanjiangyuan National Park, an important ecological barrier area in China, is of great significance for regional ecological protection, green development, and ecosystem health diagnosis. This paper analyzes the spatial and temporal characteristics of WC in Sanjiangyuan National Park from 2000 to 2022 based on the InVEST model with parameter localization correction, and identifies the driving factors affecting them using image-by-image metric correlation and residual trend methods. The results show that: (1) From 2000 to 2022, the water conservation service (WC) volume in Sanjiangyuan National Park significantly increased, averaging 9.166 billion m³ annually, with a rate of change of 101 million m³/a. The establishment of the Sanjiangyuan National Park system contributed to the improvement of the WC, and the WC was improved after the establishment of the pilot project (2016-2022), the growth rate of water source nutrient quantity is higher than that before the establishment of the pilot project (2000-2016). The localized Z parameter can be used to study the WC under similar natural conditions. (2) The spatial distribution of water yield and WC capacity in Sanjiangyuan National Park was basically consistent, showing a distribution pattern of high in the southeast and low in the northwest, with the high-value area located in the Lancangjiang River Source Park and the low-value area mainly concentrated in the northern part of the national park. The WC capacity of different land use types varied widely, with grassland being the main contributor to regional WC, and the WC capacity of different land use types has improved since the establishment of the national park pilot in 2016. The western and northern parts of the study area were mainly generally important and more important WC areas, accounting for the largest proportion of the total area, and extremely important and highly important WC areas accounted for a small proportion, mostly for the headwaters of various rivers. (3) WC in Sanjiangyuan National Park was positively correlated with precipitation and negatively correlated with evapotranspiration, with precipitation playing a dominant role, and changes in the bias coefficients excluding precipitation were significantly stronger than the bias coefficients excluding evapotranspiration. The influence of climate factors on WC declined during the period of 2000-2022, and since the establishment of the pilot national park in 2016, the promotion effect of precipitation on WC and the inhibition effect of evapotranspiration on WC have weakened. (4) Climate change was an important factor driving the enhancement of WC in Sanjiangyuan National Park. Comparing the two stages before and after the proposal of the national park, the two types of climate promotion (98.42%) and anthropogenic suppression (1.58%) were manifested in 2000-2016, and the percentage of area of different driving types in 2016-2022, in descending order, were as follows: climate promotion (82.44%) > co-inhibition (14.57%) > co-promotion (1.85%) > climate suppression (0.66%) > anthropogenic promotion (0.46%) > anthropogenic suppression (0.03%). Following the establishment of the pilot national parks, the synergistic impacts of climate-human co-promotion and single human activities gradually appeared, and this part was also the area where human activities promoted the rapid growth of WC, indicating that the construction of national parks, ecological protection projects, and forbidden animal husbandry management played a positive role in WC.