The water retention function of the forest ecosystem refers to the redistribution of precipitation through the canopy, litter layer, and the layer of soil in the forest ecosystem, which can effectively affect water retention and runoff regulation. This function is influenced by various factors including the types of forest ecosystem, climatic conditions, soil physicochemical properties, and topography, and thus shows significant spatial heterogeneity. The Dongjiang River, one of the tributaries of the Pearl River, is the main drinking water resource serving more than 40 million residents from the Hong Kong Special Administrative Region and the cities of the Guangdong province including Heyuan, Huizhou, Dongguan, Guangzhou, and Shenzhen. In recent years, with rapid population growth and economic development, the eco-environment of the Dongjiang River Watershed has been severely disturbed, which consequently has severely affected the water retention function of the forest ecosystem. In the present study, the spatial distribution of the water retention function of the forest ecosystem in the Dongjiang River Watershed was evaluated based on data on the land cover, climate, and soil, using the InVEST (Integrated valuation of ecosystem services and tradeoffs tool) model. The results showed that the total water retention in the forest ecosystem of the Dongjiang River Watershed was 47.29×10⁸ m³, with a peak value in the water retention function of 572.6 mm and a mean value of 204.15 mm. The water retention function of the forest ecosystem showed differences in spatial pattern with changes in elevation and slope. The water retention function increased initially, and then decreased with increasing elevation, with a peak mean value of 270 mm at an elevation of 900-1200 m. The water retention function of the forest ecosystem also increased with increasing slope, and reached 327.2 mm at a slope of 50, which was more than 60% of the average water retention function. At the watershed scale, the water retention function of the forest ecosystem in the middle reaches comprised the most important group, representing of 72% of total retention volume, followed by the lower reaches (17%) and upper reaches (11%). In the present study, analysis of the water retention function of the forest ecosystem in the Dongjiang River Watershed determined the boundary objects of ecological compensation, and defined the importance of ecosystem services in different geographical units. In addition, the priorities of ecological compensation were determined according to the level of importance of the ecosystem services in different geographical units, thereby improving the efficiency of ecological compensation and promoting the sustainable development of the watershed.