BACKGROUND Weight loss and decline of milk yield in Tibetan sheep was a challenge for the dairy industry in Qinghai-Tibet Plateau, which were considered to be caused by underfeeding of the sheep during the harsh winter. The objective of this study was to assess the role of feed supplementation in the milk performance and rumen microbiome of ewes under forage-based diets. Based on parity, milking period, milk yield, and body weight, ten 1.5-yr-old ewes were allocated randomly into two groups. One group of ewes was fed no supplement Control group (CON) and the other group was fed with concentrate feed supplement (Treatment group, T). Individual milk yield was determined daily; both the milk composition and rumen bacterial characteristics were analyzed after the end of feeding trials. RESULTS Results showed that lactose in the milk of the CON group was significantly lower (P < 0.05) than that of the T group at days 30 and 60. Milk yield in the T group was greater than in the CON group at day 30 (P < 0.05). Additionally, the dominant ruminal bacteria (phyla Bacteroidetes, Firmicutes, and Verrucomicrobia) were shared by both groups through 16S rRNA gene pyrosequencing. Greater relative abundance of Bacteroidales RF16 group in family level, Victivallales in order level, Lentisphaeria in class level, and Lachnospiraceae bacterium in species level were observed in the T group than in the CON group (P < 0.05). CONCLUSIONS These results demonstrated that supplementation of concentrate in the cold season improved milk lactose yield and milk production, and the rumen microbial abundance of Tibetan sheep.