Food-derived polysaccharides have advantages over synthetical compounds and have attracted interest globally for decades. In this study, we optimized the cellulase-assisted extraction of polysaccharides from white hyacinth bean (PWBs) with the aid of response surface methodology (RSM). The optimum extraction parameters were a pH of 7.79, a cellulase of 2.73%, and a ratio of water to material of 61.39, producing a high polysaccharide yield (3.32 ± 0.03)%. The scavenging ability of PWBs varied on three radicals (hydroxyl > 2,2-diphenyl-1-picrylhydrazyl (DPPH) > superoxide). Furthermore, PWBs contributed to the proliferation of three probiotic bacteria (Lactobacillus acidophilus LA5, Bifidobacterium bifidum BB01, and Lactobacillus bulgaricus LB6). These investigations of PWBs provide a novel bioresource for the exploitation of antioxidant and probiotic bacterial proliferation.