Abstract: The study was to explore the optimum processes of extraction polysaccharides from Osmunda japonica leaves and the effect of Polysaccharides on cancer cells. Using the ultrasonic microwave synergistic extraction,solid- liquid ratio,microwave power and extraction time as factors,based on the single factor experiment,the response surface methodology( RSM) Box- Benhnken center combination experimental design was employed to optimize the extraction of polysaccharides of Osmunda japonica.The results of optimum extraction conditions were as follow: the solid- liquid ratio was 1∶ 35( g / m L),microwave power 353 W,extraction time was 250 s.Under these conditions,the extraction yield was 0.236%. According to the Cytotoxicity test,the inhibitory effect of polysaccharides from Osmunda japonica on the Hep G-2 cancer cells had been studied.The results of cytotoxicity test indicated that the concentration of polysaccharides were 0.001 mg / m L,10 mg / m L separately,the inhibition rate was 18.13%,72.65% respectively.The vaule of IC50 was 0.474 mg / m L.