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Piglet weaning is an important stage in production where changes in the environment and diet can cause problems such as intestinal inflammation and diarrhea. Vitamin A is an essential nutrient for human and animal growth and has immunomodulatory and inflammatory effects. A large body of literature has previously reported on the use of vitamin A in piglet production, so our experiment added different concentrations of vitamin A (0, 1100, 2200, 4400, 8800, and 17,600 IU/kg) to weaned piglet diets to study the effects of different doses on growth performance, intestinal barrier, inflammation, and flora in weaned piglets. We selected 4400 IU/kg as the optimum concentration of vitamin A in relation to average daily weight gain, feed intake, feed-to-weight ratio, and diarrhea rate, and subsequently tested the inflammatory factors, immunoglobulin content, antioxidant levels, and intestinal flora of weaned piglets. Results: We observed that the diarrhea rate of weaned piglets was significantly lower after the addition of 4400 IU/kg of vitamin A to the diet (<i>p</i> < 0.05). A control group and a 4400 IU/kg VA group were selected for subsequent experiments. We found that after the addition of vitamin A, the serum CAT level of weaned piglets increased significantly, the expression of <i>Claudin-1</i> in the jejunum and ileum increased significantly, the expression of <i>Occludin</i> gene in the jejunum increased significantly, the expression of <i>IL-5</i> and <i>IL-10</i> in the ileum increased significantly (<i>p</i> < 0.05), and the expression of <i>IL-4</i>, <i>IL-5</i>, and <i>IL-10</i> in the ileum increased significantly (<i>p</i> < 0.05). Meanwhile, in the colonic flora of vitamin A-added weaned piglets, the relative abundance of <i>Actinobacteria</i> and <i>Erysipelotrichales</i> decreased significantly, while the relative abundance of <i>Bacteroidales</i> increased significantly (<i>p</i> < 0.05). The results of this study indicated that vitamin A at 4400 IU/kg reduces diarrhea in weaned piglets by increasing antioxidant levels, increasing intestinal tight junction protein gene expression, and regulating colonic gut microbiota.