Hyaluronic acid (HA) is a new food functional component with good antioxidant and anti-inflammatory activities. At present， most of the HA available on in the market is high molecular weight HA (HMW-HA)， whose activity is not as good as that of small molecular weight HA. To investigate the biological activity of hyaluronic acid with a specific molecular weight， HMW-HA was enzyme-hydrolyzed by a continuous enzyme-membrane separation coupling reaction system composed of an enzymatic bioreactor and a plate type polyether sulfone ultrafiltration membrane separation module， thereby low molecular weight hyaluronic acid (LMW-HA) with molecular weight of about 35 ku and oligo-hyaluronic acid (O-HA) with molecular weight of about 400 u were prepared simultaneously. The antioxidant and anti-inflammatory effects of the three HA samples with different molecular weights were then investigated in vitro and in vivo. Results showed that the two HA hydrolysates had higher scavenging abilities to DPPH free radical and hydroxyl free radical， and the survival rate of Caco-2 cells damaged by H2O₂ oxidative stress was significantly improved by 1.5 mg/mL of LMW-HA， with increased activity of antioxidant enzymes and lower levels of inflammatory cytokines. Compared with HMW-HA， ulcerative colitis in mice was better alleviated by 33.3 mg/kg of LMW-HA and O-HA， with lower disease activity score (P<0.05)， restored colon length (P<0.05)， reduced gross damage score in vivo， enhanced antioxidant enzyme activity (P<0.05)， and decreased inflammatory factors (P<0.05)， among which LMW-HA had the best antioxidant and anti-inflammatory effects. The above results indicated products of HMW-HA with moderate enzymatic hydrolysis had the best antioxidant and anti-inflammatory activities. These results provided a reference for the future application of HA in food.