Delaying skin-aging through diet is a hot research topic in recent years, but the anti-aging effects of fish gelatin and related mechanisms are not well understood. In this study, we prepared edible fish gelatin from the swim bladder of Cynoscion acoupa using three different processing methods, namely dried (DCM), soaked (SCM) and instanted (ICM), to investigate its anti-aging effects and mechanisms on D-galactose induced skin aging in mice, as well as its effects on the gut microbiota. The results demonstrated that fish gelatin significantly increased water content, collagen, hyaluronic acid (HA) and hydroxyproline (Hyp) content, and skin integrity of mice skin, as well as enhanced the antioxidative ability and anti-inflammatory capacity of the skin. In terms of protein and mRNA expression levels in skin tissue, CMs treated with different treatments can up-regulate the expression of epidermal growth factor receptor (EGFR) and tissue inhibitor of metal protease 1 (TIMP1), down-regulating the expression of matrix metalloproteinase 1 (MMP1) and matrix metalloproteinase 3 (MMP3), and increase the expression of collagen type III alpha 1 chain (COL3A1) and collagen type I alpha 2 chain (COL1A2). CMs attenuated the D-galactose-mediated inhibition collagen expression by stimulating the transforming growth factor beta (TGF-β)/Smad signaling pathway, thereby maintaining collagen matrix homeostasis. In addition, we revealed that CMs reversed gut microbiota by increase the abundance of intestinal flora. In conclusion, we demonstrated that CMs, especially for ICM, as an effective dietary supplement, have potential anti-aging and skin health benefits.