This invention provides a pharmaceutical composition for modulating immune system comprising at least one polysaccharide extracted from Radix Astragali, Radix Codonopsis, Ganoderma sinense, or mixtures thereof, and their extraction process. The present invention further relates to a method of modulating immune system by applying said pharmaceutical composition to a subject suffering from cancers, and a method of preventing and/or pretreating and/or treating cancers in the subject, where said method comprises applying said pharmaceutical composition before/during chemotherapy.