Understanding the dietary niche characteristics of Ochotona curzoniae across different population densities is of great significance for correctly recognizing its function and position in grasslands and scientifically evaluating their harmfulness. Using Bayesian stable isotope mass balance mixing model, we analyzed the dietary habits and ecological niche characteristics of four tissue types of O. curzoniae at different population densities in the alpine meadows of the Qinghai-Tibet Plateau. The liver and muscle tissues represented short-term diets, while hair and bone tissues represented long-term diets. The results showed that in the low-density areas, the diet represented by liver, muscle, and fur mainly consisted of the aboveground parts of Potentilla fragarioides and Leontopodium nanum, while the diet represented by bone was dominated by Carex alatauensis. In the medium-density areas, the peak-season diet represented by liver and muscle mainly included the aboveground parts of P. fragarioides and Oxytropis ochrocephala, while that represented by fur and bone primarily consisted of the aboveground parts of L. nanum and C. alatauensis. In the high-density areas, the peak-season diet represented by liver mainly included the aboveground parts of O. ochrocephala and P. fragarioides, while the diet represented by muscle, fur, and bone was mainly composed of the aboveground parts of Ajania tenuifolia. Overall, O. curzoniae showed considerable differences in their food selection strategies under different population densities. As population density increased, food availability decreased, leading to a shift in foraging strategy from nutritional preference to availability-driven selection.