Abstract: This study focuses on the Ailao Mountain region and aims to scientifically assess and evaluate the changes in the water conservation function of the regional ecosystem in 2000, 2010, and 2020 using the water balance method, and to provide an evaluation of its significance. We found that water conservation capacity was significant spatial different, the specific manifestation is highest in the southeast region, higher in the middle region, and northwest region was the lowest. Total water conservation fell by 30% between 2000 and 2020. The decrease of precipitation is the main factor affecting the decrease of water conservation in Ailao Mountain region. Under the background of climate warming and drying, water conservation function in Ailao Mountain region may decline further. Forest is the main contributor to water conservation in the Ailao Mountain region, with its water conservation capacity accounting for the highest proportion among various ecosystems, approximately two-thirds of the total water conservation in the study area. The grade of water conservation of Ailao mountain's ecosystem was dominated by general grade and medium grade in 2000 to 2020. The area of medium grade decreased significantly, and the general grade area increased significantly. Understanding the spatiotemporal variability of regional water conservation function provides a scientific basis for assessing the ecological environment and optimizing ecosystem management decisions in the region.