Abstract: In order to obtain the best extraction process of sulfated polysaccharides from Kappaphycus alvarezii and determine its antiallergic activity, the single factor experiments were carried out firstly. The polysaccharides yield of Kappaphycus alvarezii was chose as the investigation index, and the ratio of material to liquid, ultrasonic pretreatment time and extraction time were selected for Box-Behnken response surface design. Then the extracted crude polysaccharides was purified through the DEAE-52 cellulose chromatography column. The effects of the purified polysaccharides on RBL-2H3 cell viability, inhibition of cell degranulation and regulation of histamine release were measured by RBL-2H3 cell model method. The results showed that the optimum extraction condition of polysaccharides from Kappaphycus alvarezii was as follows: The ratio of material to liquid (w/v) was 1:92 g/mL, the ultrasonic pretreatment time was 40 min, and the extraction time was 4 h. Under the optimum extraction conditions, the extraction rate of polysaccharides from Kappaphycus alvarezii was 13.92%. The KSP（Kappaphycus alvarezii Sulfated Polysaccharides）, a component purified by DEAE-52 cellulose chromatography, had no significant inhibition on the activity of RBL-2H3 cells in the concentration range from 50 to 1200 μg/mL, and also had no cytotoxicity. The β-Hexosaminidase activity and histamine release inhibition rates of RBL-2H3 cells stimulated by 2,4-Dinitrophenyl-Bovine Serum Albumin (DNP-BSA) with 25, 50 and 100 μg/mL KSP were 36.3%, 37.0%, 51.7%, and 17.5%, 18.3%, 42.0% respectively. In conclusion, the process parameters obtained in this test are feasible, KSP had no obvious toxicity to RBL-2H3 cells and showed significant inhibition of degranulation and histamine release, with good in vitro anti-allergic activity. This study can provide a reference for the development of nutritional functional food with anti-allergic activity using Kappaphycus alvarezii as raw material.