Abstract: Ultrasonic technology was used to assist the extraction of fucoxanthin from Sargassum thunbergii. The optimum conditions were determined by using single factor experiment and response surface methodology. The high performance liquid chromatography(HPLC)and electrospray ionization mass spectrometry(ESI/MS)method were developed for the analysis of fucoxanthin that was isolated and purified by silica gel column. The experimental results showed that the optimum ultrasonic extraction model was as follows:Ethanol concentration 80%(v/v),ultrasonic starting temperature 52 ℃,extraction time 32 min,and solvent to material ratio 10 mL/g,the dosage of L-ascorbic acid 0.8%,extract two times,the fucoxanthin extraction yield was(0.455±0.046) mg/g. After purification,the purity of the fucoxanthin isolated was about86.88%±1.34%,and the extraction yield was(0.048±0.002) mg/g. This method that prepared the high purity fucoxanthin was simple and the extraction ratio was high,which would provide experimental basis and technical support for its comprehensive utilization in the future.