Abstract: Objective:To optimize extracting conditions of phlorotannins from Sargassum horneri using ultrasonic assisted extraction,making a preliminary structural analysis on the phlorotannin extraction. Method:Single-factor test and response surface methodology were applied to optimize three parameters,which were ethanol volume fraction,solid-liquid ratio and ultrasonic extraction time,for extracting phlorotannins from Sargassum horneri using the ultrasonic assisted extraction method. The content of total phenols in brown algae extracts was measured by Folin-Ciocalteu method,whilst that of phlorotannins were measured by 2,4-dimethoxybenzaldehyde assay(DMBA assay),and the correlation between the two was analyzed. The molecular formula of compounds in the phlorotannin extract was identified by LC-TOF-MS before speculating their structures. Result:The response surface interaction analysis showed that the optimum extraction conditions were ethanol volume fraction at 55%,liquid-solid ratio at 42:1 mL/g,extract time at 58 min. In this condition,the extraction yield of phlorotannins from Sargassum horneri was(25.16±0.34) mg/100 g. The yield of phlorotannins measured by DMBA assay was positively correlated with the total phenol yield measured by the Folin-Ciocalteu method.(The correlation coefficient 0.803)One compound in the extract was identified with molecular weight being 374.3036,and its molecular formula as C₁₈H₁₄O₉. The trimer structure was assumed to be Triphlorethol A. Conclusion:The extraction yield of phlorotannins from Sargassum horneri was improved. The yield of phlorotannins and the yield of total phenols in Sargassum horneri extracts were positively correlated. Sargassum horneri contained phlorotannins Triphlorethol A.