Gastric ulcer is a peptic ulcer disease caused by gastric mucosal lesion. Because of its various side effects of chemotherapy drugs, it is very important to seek safer and more effective natural active compounds to treat gastric ulcer. Polysaccharides were extracted from small black soybean, cowpea, pea and chickpea by water extraction and alcohol precipitation. The basic nutritional composition, monosaccharide composition and molecular weight distribution were determined, and the protective effects and potential mechanism of four legume polysaccharides on ethanol-induced acute gastric mucosal lesion in mice were explored. The results showed that the sugar contents of polysaccharides from small black soybean, cowpea, pea and chickpea were 66.40%, 71.79%, 67.41% and 53.31%, respectively, and the molecular weight distribution was wide. The monosaccharide composition of cowpea, pea and chickpea polysaccharides were mainly arabinose, galactose and galacturonic acid, which belonged to pectin acidic polysaccharide, while the small black soybean polysaccharides were mainly composed of galactose and mannose, which belonged to neutral polysaccharide. The inhibition rates of small black soybean, cowpea and chickpea polysaccharides on gastric mucosal lesion were 37.1%, 34.4% and 29.8%, respectively, which could reduce the gastric mucosal lesion in mice. Cowpea polysaccharides could significantly inhibit the expression of interleukin-6 and tumor necrosis factor-α (P < 0.05). Small black soybean polysaccharides could significantly reduce the content of malondialdehyde in gastric tissue and increase the activity of total superoxide dismutase and the content of reduced glutathione (P < 0.05). Both two polysaccharides had better gastric mucosal protective effect. This study wished to provide a theoretical reference for the development of healthy food with gastric protection function by using legume polysaccharides.