Supplemental feed for free-ranging deer is often advocated by managers to compensate for winter survival. However, providing extra food has great costs so it is important to know how essential supplemental feed is for deer populations. In our study, supplemental feed is supplied year round for free-ranging Père David’s deer (Elaphurus davidianus) since early 2019 to help them survive food shortages in the Jiangsu Dafeng Milu National Nature Reserve. To understand use of supplemental feed, we quantified the biomass contributions of C3 plants, C4 plants, and supplemental feed consumed by free-ranging deer by assessing their faecal carbon and nitrogen stable isotope values in May, July, October and December. The diet-faeces trophic shift of deer was assessed using semi-captive animals. Faeces in both areas were collected and faecal δ¹³C values revealed more variable diets of free-ranging deer than semi-captive deer in different months. MixSIAR model showed that C4 plants contributed greatly to deer diet biomass (75.1%, 82%, 64.3%,79.4%, respectively in May, July, October and December) compared with C3 plants and supplemental feed. We suggest that managers should consider the efficiency of long-term supplemental feed and consider the selection of C3 and C4 plants during restoration.