This is the first academic report about extracting polysaccharides from Halymenia sinesis (H. sinesis). H. sinesis and Grateloupia filicina (G. filicina) belong to the same genus, but the study of G. filicina polysaccharides was recorded frequently. With the methods of single factor test and L₉ (3³) orthogonal experiment, this paper studied how extracting time and extracting temperature affect the extraction of soluble polysaccharide from H. sinesis when using ultrasonic-assisted skills. This paper aims to establish an optimum technique and form the operational procedures of extracting polysaccharides from algae and provide a basis for pilot batches and large-scale production. Experimental results showed that the optimizing of extracting time and extracting temperature were 4 h and 90℃, and the concentration of raw material to water (CMW) would significantly affect the extraction percentage of polysaccharides. In addition, CMW was the main factor affecting the extraction percentage of polysaccharide, ultrasonic time was the second factor and ultrasonic power the third. After a set of effective experiments, the extraction yield of polysaccharide could reach 36.91%, and the content of total sugar was up to 63.85% on the conditions of CMW 0.020 g/mL, ultrasonic time 60 min and ultrasonic power 400 W.