Abstract: The Beijing Plain Afforestation Project has enhanced the ecological carrying capacity of the city and optimized the ecological spatial pattern of the capital. However, the cost-effectiveness of this project remains unknown. This study, based on typical plot surveys and the i-Tree Eco model assessment, analyzed and predicted the ecosystem service value of the Beijing Plain Forest from 2014 (the year following afforestation) to 2050. The aim was to systematically quantify the ecosystem service value and related costs of the Beijing Plain Forest, and to reveal the long-term investment return of the Beijing Plain Afforestation Project. The results show that the project has achieved good ecological benefits. During 2014 to 2050, the average value of the ecological benefit per unit area of the plain forest in the study area was 143 309.14 yuan/(hm²·a), and the average annual benefit per tree was 224.01 yuan. The average benefit-cost ratio for each year was 2.92. Considering the afforestation cost and discount rate, the net present value of the plain afforestation project was 27 606.68 yuan/(hm²·a). The forest health and wellness value, discount rate, forest land management cost, land transfer cost, carbon sequestration and oxygen release value, and tree mortality rate were the main factors affecting the net present value of the afforestation project. The research method can provide a reference for the cost-benefit evaluation of the Beijing Plain Afforestation Project, with the aim of providing decision support for the sustainable management of the Beijing Plain Forest.