Bush encroachment is widely distributed in arid and semi-arid regions, and it has a serious impact on livestock production, especially in Africa where livestock is a primary source for the livelihoods of many people. In this study, methods of supervised classification and decision tree classification, and indexes of a land use change significance index (Ci) and a single land use dynamic degree, were applied to remote sensing imagery of Ethiopia for 1986–2016. The results show the dynamic characteristics of grassland bush encroachment in low altitude areas (pastures <1500 m above sea level) of Ethiopia in the 30 year period studied. The results revealed several interesting features of this phenomenon. (1) The area of bush encroachment showed an increasing trend, with a maximum of 3.74×10³ km² in 2003, which represented 68.97% of the total area, and declined slightly from 2003∼2016. (2) Among classification types, the area of severe shrub grassland was the largest, accounting for 28.36–49.10% of the total area, while the area of moderate bush encroachment accounted for 9.77–16.68%, and slight bush encroachment accounted for 5.52–7.57%. (3) The expansion rate of shrubby grassland was 0.74% for the 30 year period, while the average annual expansion rate was 2.16% for the 8 years from 1995–2003. (4) Forest land and grassland were the two main land use types of shrub grassland transformation in low altitude areas and bush encroachment changed large amounts of forest land into grassland due to shrub grassland management. The results of this study provide basic data for revealing the impacts of development processes on subsequent bush encroachment and can inform better management for the sustainable development of grasslands in low altitude systems.