Diet selection in livestock is a complex physiological and ecological adaptation process. It is also a core problem in terms of the foraging behavior of grazing herbivores, which is of considerable importance for sustainable grassland management. To date, the available studies have predominantly focused on the effects of plant community composition, structure, and individual characteristics of grazing herbivores on diet selection. However, the study on responses of forage plants in the context of different topographies to the diet selection of grazing herbivores is rare. In this study, forage plants, comprising gramineae, cyperaceae, leguminosae, forbs, edible shrubs and the whole plant community on different topographies, that is, terrace, riparian zones, shady slope, half shady slope, half sunny slope, sunny slopes in the Qilian Mountains were selected. Changes in the nutritional and mineral content of forage plants and differences in Tibetan sheep forage preference for forage plants on different topographies were examined using an indoor cafeteria trial. A structural equation model was used to illustrate the effects of forage plants on different topographies on Tibetan sheep forage preferences. Our results demonstrated that: 1) the nutritional value of gramineae was the highest on the shady slope. Meanwhile, that of cyperaceae and leguminosae was the highest on the terraces. The nutrient value of forbs and the whole community was the highest on the sunny slope. 2) The dry matter intake of Tibetan sheep to gramineae, cyperaceae, forbs and the whole plant community was the highest on the sunny slope (42.6, 68.7, 69.8 g/h and 237.6 g/h, respectively), and the lowest on the riparian zones (7.3, 8, 21.8 g/h and 69 g/h, respectively). 3) Tibetan sheep forage preference of gramineae, cyperaceae, forbs and community were the highest on the sunny slope, the forage preference index was 5, 8, 8.1 and 27.7, respectively; and the lowest on the riparian zones. 4) Structural equation model (SEM) showed that the crude fiber and crude protein content of forage plants were dominant factors affecting forage preference of Tibetan sheep. The results provide a scientific basis for understanding the relationship between livestock and plant in grassland and formulating scientific grazing management strategies, and is of great significance for the sustainable utilization of grassland-livestock husbandry.