[Objective] The ecological water requirement (EWR) of vegetation in different typical zones for the rational allocation of water resources in the Inner Mongolia Autonomous Region was analyzed and calculated to provide scientific suggestions for water resources management and ecological protection in this region. [Methods] Based on the Penman-Monteith formula, this study used measured and multi-source remote sensing data to invert EWR and the ecological water requirement per unit area (ET) for vegetation in four typical regions (western, central, eastern, and northeastern regions) of the Inner Mongolia Autonomous Region from 1990 to 2020. The dominant factors of spatiotemporal variation of ET were determined. [Results] ① EWR showed a trend of decreasing and then increasing over the 30-year study period, with the western region being more stable (maintaining an EWR of around 3.00×10¹⁰ m³). The central and eastern regions had EWR of around 6.00×10¹⁰ m³, with a large variation and a maximum variation of over 1.00×10¹⁰ m³ between years. The northeastern region EWR values were higher and stable at around 1.00×10¹¹ m³. ② EWR was higher in the east and lower in the west, and varied greatly from east to west, with the mean ET value in the northeastern region being nearly 250 mm higher than in the west. [Conclusion] The climatic factors (primarily precipitation and evaporation) and anthropogenic factors (primarily land cover type change) were the key factors for the change of ecological water requirement of vegetation in the Inner Mongolia Autonomous Region, with the latter being more influential than the former.