Abstract: Continuous deterioration of Mongolia’s ecological environment and weak ecological barrier along the China-Mongolia border are common development problems facing the two countries． However, at present, we lack quantitative assessment of the type, characteristics and driving factors of comprehensive and multi-factor coordinated development of social economy and eco-environment at the national scale, with adverse effect on overall promotion of the green development pattern of Belt and Road between China and Mongolia． In this paper we summarize the present status of socio-economic development and eco-environment development in Mongolia, and use AHP-CRITIC- game theory to calculate subjective and objective combined weights of index elements, and comprehensively evaluate the coupling coordination degree of socio-economic development and eco-environment development in Mongolia． We find that from 2005 to 2021, the coupling coordination degree of socio-economic development and eco-environment development in Mongolia presents a fluctuating upward trend, and the coupling coordination degree type always belongs to the seriously imbalance type, lagging development of ecological environment is the characteristic of the current development stage． Level of social development and endowment of ecological environment are the main driving factors for the socio-economic subsystem and eco-environment subsystem． Pearson correlation analysis shows that mismatch between decrease of water area and forest resources, deterioration of water quality and increase of disaster/accident response such as dust weather and steady development of economy and society may be the reason for the formation of this feature and the low coupling coordination degree． To deepen synergy of bilateral green development strategies, China and Mongolia should, while promoting exchanges in socio-economic development, deepen exchanges and cooperation in strengthening eco-environment protection and restoration efforts to restore eco-environment endowments．