The evaluation of ecosystem service functions is a hot issue across the world as it is related to human well-being. Water conservation function has become one of the most important service functions and plays a key role in ensuring regional ecological security and sustainable development. Based on the InVEST model, this study quantitatively evaluated the water conservation function of Heilongjiang Province from 2000 to 2020. We analyzed the dominant influencing factors of the spatial differentiation characteristics of water conservation function changes in the past two decades through geographical detector, and combined geographically weighted regression to capture the spatial variation of the driving factors. The results showed that:(1) the average water conservation amount of the province increased from 13.81 mm/m² to 20.29 mm/m² from 2000 to 2020, and the total water conservation amount increased by 46.92%. (2) Climate change was the main driver of water conservation change, while land use change was an indirect driver. The interaction between climate and land use had remarkably greater impacts on the spatial variability of water conservation changes than any single factor. (3) The correlation and intensity of the drivers and water conservation change had obviously spatial differentiation characteristics. The change of water conservation was positively correlated with precipitation and forest and grass coverage in the whole province. The change of water conservation was negatively correlated with the coverage of the cultivated land and urban-rural industrial-mining land in the Xinganling Mountains and the Three Rivers Plain. This study can provide a scientific basis for management departments to make ecological protection decisions and optimize ecosystem management.