The invention belongs to the field of Chinese medicine, relates to Prunella vulgaris polysaccharide, and a preparation method and purpose thereof in preparing anticomplement medicines. The Prunella vulgaris polysaccharide including uniform polysaccharide PW-PS1 and PW-PS2 is prepared from a dry fruit aqueous extract of a labiatae plant Prunella vulgaris Linn. Through in vitro tests, the uniform polysaccharide is proved to have strong anticomplement activity and inhibition effects on classical pathway and alternative pathway of a complement system; besides, the polysaccharide does not have the anticoagulant effect influencing in vivo activity and can be used for the preparation of complement inhibition medicaments. The PW-PS1 effects on C1q, C3 and C9 components of the complement system, and the PW-PS2 effects on the C1q, C2, C3, C5 and C9 components of the complement system.