Feeding Regel () could improve lamb growth, immunity, and antioxidant capacity. These effects were supposed to be mediated by the rumen microbiota, as reported in our previous studies, but further verification is required. The purpose of this study was to verify whether changes in serum immunity, antioxidant, and biochemical indicators of meat sheep mediated by AMR are achieved via rumen microbiota regulation. The experiment included two phases. In phase I, twelve 90-day-old male lambs (25 ± 1 kg) were used as rumen fluid donors and consumed a basal diet with 15 g/day AMR for 135 days to induce changes in their rumen microbiota. In phase II, thirty 90-day-old male lambs (23 ± 2 kg) were split into three groups (n = 10 each): the control group () fed the basal diet; the AMR-supplemented group () fed the basal diet supplemented with 15 g/day of AMR; and the rumen fluid recipient group () fed the basal diet and received rumen fluid transplantation. The CON and AMG groups received four oral infusions of 250 mL saline, while the RTG group received four oral infusions of 250 mL donor rumen fluid. Phase II lasted for 75 days, and the blood samples were collected on the last day. Rumen fluid transplantation was performed every 15 days, with a total of four infusions of 250 mL each. The results showed that the final body weight and average daily gain () of the AMG and RTG groups were higher than those of the CON group ( < 0.05), while there were no significant differences between the AMG and RTG groups ( > 0.05). On day 30, the levels of interleukin-10 (), immunoglobulin A (), immunoglobulin M (), and immunoglobulin G () in the AMG and RTG groups were increased ( < 0.05), and malondialdehyde () was significantly decreased ( < 0.05), with no significant differences between the AMG and RTG groups. On day 60, total antioxidant capacity (), IgM, IgG, and catalase () were increased in the AMG and RTG groups, while tumor necrosis factor alpha (), low-density lipoprotein cholesterol (), and blood urea nitrogen () were decreased ( < 0.05), and there were no significant differences between the AMG and RTG groups ( > 0.05). High-density lipoprotein cholesterol () in the RTG group was significantly lower than in the CON and AMG groups, while the AMG group had significantly higher HDL-C than the CON group ( < 0.05). Microbial analysis indicated that , , and were positively correlated with IgM; , 791, , , , and 7741 were negatively correlated with TNF-α; 791 was positively correlated with T-AOC; was positively correlated with CAT; MDA was negatively correlated with ; and 7741 were negatively correlated with HDL-C; and , , , and 7741 were negatively correlated with LDL-C. Therefore, the study indicates that both supplementation with AMR and transplantation of rumen fluid from sheep fed with AMR can enhance the immunity and antioxidant capacity of lambs by increasing the abundance of the aforementioned bacteria. It also verified that the improvement in immunity and antioxidant capacity mediated by Regel is driven by the rumen microbiota.