Abstract: In order to intuitively reflect the impact of the Nature Forest Conservation Project (NFCP) on the conservation of water and soil function in forest ecosystems and present the ecological benefit, according to the assessment standards of the People’s Republic of China(GB/T 38582-2020 Specifications for assessment of forest ecosystem service in China), the researchers in this paper have taken the key state-owned forest districts in Inner Mongolia Area of China as examples to quantitatively analyze the conservation of water and soil function in forest ecosystems of two stages, including Before NFCP (1988) and After NFCP (2018). The results show that the forest ecosystem in key state-owned forest districts in Inner Mongolia Area of China has an annual value of 950.16×10⁸ yuan in 1998 and 1 341.32×10⁸ yuan in 2018 for water conversation, 563.49×10⁸ yuan in 1998 and 760.11×10⁸ yuan in 2018 for soil conversation. The top three forest bureaus with highest values of forest water conversation before and after NFCP are Genhe, Moerdaoga and Wuerqihan, while the top three forest bureaus with highest values of soil conversation are Genhe, Wuerqihan and Jinhe. The spatial pattern distribution of these two conversation values generally decreased from middle and northern parts to southern part.