The Optimization of the extraction of polyphenols from Porphyra haitanensis and their antioxidant and antimicrobial activities in vitro were studied. The effects of liquid-material ratio, ultrasonic times and ethanol concentration on the contents of polyphenols from P. haitanensis were investigated. Moreover, the response surface analysis method was applied to determine the optimization of the extraction. The results showed that the optimum condition for the ultrasonic-assisted extraction of polyphenols was as follows: liquid-material ratio 35∶1 (mL/g), ultrasonic times 43 min and ethanol concentration 66%. Under the optimum condition, the contents of polyphenols was (6.85±0.13) mg GAE/g. The research of their antioxidant activity in vitro showed that the polyphenols had the scavenging capabilities of DPPH and ABTS radicals, and their IC₅₀ were (36.67±0.75) and (16.07±0.32) μg/mL, respectively. The value of oxygen radical absorbance capacity (ORAC) was (1 005.1±11.8) μmol TE/g. Meanwhile, their antimicrobial activity in vitro showed that the polyphenols could inhibit the growth of tested microorganisms including Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Sarcina lutea, and their minimal inhibitory concentrations were 1, 1, 1， 2 mg/mL, respectively.