Abstract: Objective: The anti-aging effect of enzymatic hydrolysate extract of Ulva lactuca L. on D-galactose-induced aging model mice was studied. Methods: D-galactose was subcutaneously injected into the neck and back of mice at a dose of 800 mg/kg·d for 6 weeks to establish a subacute aging model. After 28 days of administration of the extract and Vitamin C, the skin aging and oxidative stress levels of the mice were characterized. Results: Compared with the water extract, the enzymatic hydrolysate extract of Ulva lactuca L. showed significantly higher levels of soluble total sugars, proteins, reducing sugars, and polyphenol solids (P<0.05), and increased the clearance rates of hydroxyl and DPPH free radicals and superoxide dismutase (SOD) activity by 5.72, 7.95, and 13.26 times, respectively. Animal experiments showed that the enzymatic hydrolysate extract of Ulva lactuca L. significantly improved the glutathione peroxidase (GSH-Px), total superoxide dismutase (T-SOD), catalase (CAT), total antioxidant capacity (T-AOC), and malondialdehyde (MDA) and protein carbonyl levels in aging skin, and alleviated organ atrophy in mice. The collagen content in the skin increased by 71.63%, and the ratio of type I/III collagen increased by 55.40%. The enzymatic hydrolysate extract of Ulva lactuca L. significantly increased the GSH-Px, T-SOD, and CAT activity and T-AOC level in the body, and inhibited the oxidative degradation of biomolecules in the body, such as reducing protein carbonyl and MDA levels (P<0.05). Conclusion: The enzymatic hydrolysate extract of Ulva lactuca L. can reduce oxidative stress levels in mice and delay skin aging.