An accurate assessment of the ecosystem service is a fundamental prerequisite for achieving sustainable regional development in arid and semi-arid regions. Current indices for evaluating ecosystem services typically aggregate the cumulative or maximum values of a specific set of ecosystem services. However, when the performance of individual ecosystem services varies widely, these indices can be disproportionately influenced by outliers. This may lead to either overestimation or underestimation of the overall ecosystem services. This study proposes a novel assessment approach, namely the multi-dimensional ecosystem service index (MDESI), with a view to addressing the aforementioned issues. And the MDESI is employed to assess ecosystem services in a typical arid and semi-arid region, exemplified by Inner Mongolia from 2001 to 2020. The results demonstrate that (1) in arid and semi-arid regions, the MDESI is more effective than the traditional cumulative and maximum methods; (2) from 2001 to 2020, the ecosystem service capacity of Inner Mongolia exhibited a spatial pattern of decline from the northeast to the southwest; (3) despite regional variations, soil and water conservation services represent the most significant constraint on ecosystem services in Inner Mongolia. This study provides a theoretical basis for the government to formulate sustainable development.