Food source analysis is the foundation of the research of Elaphurus davidianus habitat demands in Elaphurus davidianus conservation, and it is necessary and essential to comprehend the interaction of Elaphurus davidianus and their abiotic environment, carrying capacity evaluation, and manage resource management. The study selected the first and third core areas of the protection zone as the research areas, conducting field surveys in May, July, October, and December 2019 respectively. Samples of Elaphurus davidianus' edible plants and feces were collected, and using the carbon and nitrogen stable isotope technique, six edible plants were quantified: mutualistic Spartina alterniflora, Suaeda glauca, Imperata cylindrica, Pennisetum alopecuroides, Cynodondactylon, Carex scabrifolia and deer excrements, and the correlation between food sources and the excrements were analyzed. The Bayesian model was also used to determine the proportional contributions of these plants to the diet of Elaphurus davidianus, as well as the variation of the feeding preference of Elaphurus davidianus in different seasons. The study found that the Elaphurus davidianus foraging behavior was affected by the type, quantity, and nutritional value of edible plants. The highest contributions to the Elaphurus davidianus food supply were discovered to come from Spartina alterniflora, Cynodondactylon, Spartina alterniflora, and Spartina alterniflora in the months of May, July, October, and December, with the contribution rates of 39.0%, 42.8%, 47.9%, and 31.9% respectively. The significant variation in the contribution of Cynodondactylon and Spartina alterniflora to the Elaphurus davidianus' diet in May, July, October, and December suggests that the Elaphurus davidianus' foraging preferences differed in different seasons. Elaphurus davidianus on the reserve were found to consume a mix of C₃ and C₄ plants; although the proportion of C₄ plants was greater than that of C₃ plants, whether the Elaphurus davidianus preferred the C₄ plants cannot be determined. Spartina alterniflora was a significant food supply for Elaphurus davidianus in the third core area under conditions of supplemental feeding, while Pennisetum alopecuroides had a lower contribution rate, indicating that Elaphurus davidianus population prefer plants with higher nutritional value as their food. The variations in δ¹³C and δ¹⁵N values of deer feces in different months indicated that Elaphurus davidianus adjust their food components seasonally to meet their nutritional needs. Understanding the dietary dynamics of Elaphurus davidianus is not only crucial for data support for the development of effective management and protection strategies, but a scientific basis for the management of Elaphurus davidianus reserves and the restoration of their habitats.