The optimum enzymatic conditions of extracellular polysaccharide of Porphyridium creuntum (EPS) were investigated in this research. Based on the results of single factor experiment, by using the Box-Behnken response surface method array design, the optimum conditions for hydrolyzing EPS to obtain antioxidant hydrolysates with the highest hydroxyl radical scavenging activity were as follows: enzymolysis reaction time 6.3 h, enzymolysis reaction pH 8.0, enzymolysis temperature 63 ℃, and enzyme-to-substrate ratio 2.7 U/mg. Under the conditions, the scavenging rate of hydroxyl radical was 70.64%. Subsequently, the antioxidant activity of the enzymatic hydrolysates were determined in vitro. The results showed that the enzymatic hydrolysates had good scavenging abilities on ·OH, DPPH·, ABTS⁺· and O^-₂·, the half-scavenging concentration values of the crude hydrolysates against ·OH, DPPH·, ABTS⁺· and O^-₂· radicals were (1.67±0.58)mg/mL, (1.08±0.01) mg/mL, (0.64±0.01) mg/mL and (4.16±0.01) mg/mL respectively.