Due to the increasing costs of livestock farming, it is important to find cost-effective alternatives of feed stuffs. This study investigated the effects of high-moisture ear corn (HMEC) feeding on the production performance, serum antioxidant capacity, immunity, and ruminal fermentation and microbiome of dairy cows. Thirty pluriparous Chinese Holstein cows were randomly allocated to two groups: steam-flaked corn (SFC) and HMEC (replacement of 2 kg equal dry matter SFC) and fed for a 60 day trial. The results showed replacing SFC with HMEC significantly increased dry matter intake, milk yield, and 4% fat-corrected milk yield (p < 0.05). Serum levels of superoxide dismutase, glutathione peroxidase, and immunoglobulins G, M, and A were significantly higher, and those of creatinine and cholesterol were significantly lower, in the HMEC group than in the SFC group (p < 0.05). HMEC also significantly increased total volatile fatty acid and acetate (p < 0.05) concentrations. In both groups, the dominant phyla of ruminal bacteria were Bacteroidetes, Firmicutes, and Actinobacteria, and the dominant genera were Prevotella, NK4A214-group, and Succiniclasticum. Mogibacterium, Eubacterium nodatum group, norank-f-Lachnospiraceae, and Eubacterium brachy group were significantly enriched in the ruminal fluid of HMEC-group cows (p < 0.05). In conclusion, replacing SFC with HMEC improved production performance, antioxidant capacity, and immunity, while regulating both ruminal fermentation and the composition of the ruminal microbiome in dairy cows.