A water-soluble polysaccharide from Chaenomeles lagenaria (CLWSP-1) was purified and structurally identified. Then, response surface methodology (RSM) was performed to optimize the hot water extraction (HWE) of CLWSP-1. In addition, the antibacterial activity of CLWSP-1 was also evaluated. The results indicated that the polysaccharide CLWSP-1 mainly contained galacturonic acid, arabinose, and galactose, and its molecular weight was 1.23 × 102 kDa. The optimal HWE for extraction of CLWSP-1 was a ratio of water to solid of 48.9 mL/g, temperature of 91 °C, and an extraction time of 114 min; an ethanol concentration of 81% and a 1.24% concentration of CLWSP-1 were achieved. Moreover, the obtained CLWSP-1 had strong antibacterial activity when exposed to Escherichia coli and Staphylococcus aureus, suggesting that CLWSP-1 may potentially contribute to the development of a natural preservative in the food industry.