The aim of this study was to determine the impact of stocking density on the liver proteome and cecal microbiota of Peking ducks. A total of 1,200 ducks with 21-day old were randomly allotted into 5 stocking density groups of 5, 6, 7, 8 and 9 ducks/m2, with 6 replicates for each group. At 40 days of age, duck serum and pectorals were collected for biochemical tests; liver and cecal contents of ducks were gathered for proteome and microbiota analysis, respectively. Serum MDA increased while pectorals T-AOC reduced linearly with enhancing stocking density. Duck lipid metabolism was altered under different stocking density as well. Serum LDL-C increased linearly with increasing stocking density. Proteome analysis revealed fatty acid biosynthesis proteins such as acyl-CoA synthetase family member 2 and fatty acid oxidation related proteins including acyl-CoA dehydrogenase long chain and acyl-coenzyme A oxidase were enriched in high stocking density group. Additionally, high stocking density increased oxidative response related proteins such as DDRGK domain containing 1 while diminished anti-oxidant capacity related proteins including regucalcin and catalase. 16S rDNA analysis revealed that higher stocking density was accompanied with decreased microbial diversity, as well as depletion of anti-inflammatory bacterial taxa, including Bacteroidales, Butyricimonas and Alistipe. In addition, decreased bile acid metabolism-associated bacteria such as Ruminococcaceae, Clostridiales and Desulfovibrionaceae were found in the high-density group. Both proteome and 16S rDNA results showed inflammation and chronic liver disease trend in the high-density group, which suggests the involvement of the liver-gut axis in oxidative stress.