The molecular weight (Mw) of hyaluronic acid (HA) is closely related to its bioactivity, but traditional degradation methods suffer from significant loss of activity and high costs. To achieve green and efficient regulation of Mw while enhancing functional diversity, this study used high-Mw HA (~1300 kDa) as the raw material and prepared low-Mw fermented HA (FHA) through mixed fermentation with Lactobacillus fermentum and Lactobacillus acidophilus. The contents of HA, total sugar, reducing sugar, polysaccharides, and total protein in FHA were determined using the carbazole method, phenol-sulfuric acid method, DNS microplate assay, and Bradford protein assay, respectively. The molecular weight distribution of FHA was analyzed by high-performance gel filtration chromatography (HPGFC). The bioactivity of FHA was evaluated based on antioxidant, anti-inflammatory, and skin barrier repair assays, while its biosafety was assessed using the chick chorioallantoic membrane (CAM) test. The results showed that the Mw of FHA was significantly reduced to approximately 4×105 Da, with total sugar, reducing sugar, polysaccharides, and total protein contents of 416.84, 870.79, 379.75, and 3.45 mg/g, respectively. At 200 μg/mL, FHA exhibited significant antioxidant and anti-inflammatory effects at the cellular level: it increased catalase (CAT) activity to 11.14 units/mg protein and reduced malondialdehyde (MDA) release to 5.50 μmol/mg protein in a UV-induced oxidative stress model, while significantly suppressing the secretion of inflammatory factors (TNF-α, IL-1β, IL-6, and IL-8). FHA at 200 and 400 μg/mL effectively mitigated the degradation of COL-I and Claudin-1 proteins in UV-damaged cells. Furthermore, 5 mg/mL FHA showed no irritation in the CAM test, confirming its safety and suitability for topical cosmetic applications.