The establishment of gut microbiota in young ruminants has a profound impact on their productive performance in adulthood. The microbial communities of ruminants differ significantly across the different regions of the digestive tract, as well as between the mucosa and lumen. In this study, we analyzed the characteristics of the microbiota of the small intestine (jejunum and ileum) and large intestine (cecum and colon) of sika deer on day 1 (birth), day 42 (transition period) and day 70 (rumination period) using 16S rRNA gene sequencing. The results showed that the microbial diversity of the mucosa in the jejunum, ileum, cecum and colon of sika deer was higher on day 70 than on day 1, and the diversity of the cecal mucosa was significantly higher than that in the small intestine. Principal coordinates analysis (PCoA) showed that the microbial community structures of the small and large intestinal mucosa were significantly different, and the microbial community structure of sika deer on day 1 was significantly different from that on days 42 and 70. The relative abundances of