The optimization of culture temperature, initial medium pH, and carbon and nitrogen sources for improved production of extracellular polysaccharides by Lactobacillus plantarum XN1904E was carried out using one-factor-at-a-time (OFAT) and orthogonal array design methods. The results showed that the optimal culture conditions were determined as follows: initial pH 6.5, temperature 37 ℃, galactose as the best carbon source, and fish peptone as the best nitrogen source. Under these conditions, the production of extracellular polysaccharide was 235.41 mg/L. The polysaccharide at 0.2 mg/mL had antioxidant activity in terms of scavenging rate (27.54%) against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical.