Abstract: This study was designed to investigate the effects of low-protein feed supplemented with methionine on the growth performance, serum biochemical indices, liver health, and intestinal flora of Monopterus albus. A total of 600 eels weighing around 25.00 g were randomly assigned to four treatments with three replications each: FM (42% protein), LP (36% protein), Met1 (LP with 0.25% methionine), and Met2 (LP with 0.50% methionine) and fed for eight weeks. The results showed that: (1) Compared with the FM group, the weight gain rate, crude protein, and crude fat content of M. albus in the LP group decreased significantly (P<0.05), together with a significantly increased feed coefficient (P<0.05). However, dietary supplementation with 0.50% methionine improved the growth performance relative to the LP group, coupled with the increases in the intestinal amylase, lipase, and protease activities (P<0.05), as well as lowered the feed coefficient (P<0.05). (2) Compared with the FM group, the serum levels of blood ammonia, urea nitrogen, alanine aminotransferase, alkaline phosphatase, acid phosphatase, and aspartate aminotransferase increased significantly in the LP group (P<0.05), which were lower in the Met2 group (P<0.05). (3) Compared with the FM group, eel fed low-protein diets exhibited lower catalase, reduced glutathione, and total antioxidant capacity in the liver (P<0.05), while malondialdehyde content significantly increased (P<0.05), indicating oxidative stress and structural liver damage. In contrast, the Met2 group demonstrated improved liver antioxidant capacity, intact hepatocyte morphology, and reduced vacuolation ratio compared to the LP group. (4) The LP group exhibited a significant increase in the relative abundance of Staphylococcus, Mycobacterium, and Methylosinus, alongside a significant decrease in Acinetobacter compared to the FM group (P<0.05). In the Met2 group, the relative abundance of Staphylococcus and Methylocystis significantly decreased (P<0.05), while Mycobacterium significantly increased compared to the LP group (P<0.05). In summary, low-protein feed contributed to growth decline, liver damage, and intestinal microecological imbalance of M. albus. Supplementation with 0.5% methionine effectively maintained liver histomorphology, enhanced growth performance, and improved intestinal health of M. albus.