Abstract: The conditions for polysaccharides extraction from Sargassum horneri were optimized with response surface analysis, and the effects of the extract on the moisture retentivity of cut tobacco were investigated. The results showed that:1) The optimal conditions for extracting polysaccharides from Sargassum horneri were:water-material ratio 56.7 mL/g, ultrasonic power 480 W, extraction time 25 minutes and duty ratio 1:1. The yield of polysaccharides was 10.82%. 2) The polysaccharides of Sargassum horneri could reduce the rate and velocity of moisture desorption of cut tobacco in low humidity condition and effectively slow down the excess moisture absorption of cut tobacco at high humidity condition, which improved moisture retentivity and humidity resistance of cut tobacco.