The invention relates to spirulina phatensis polysaccharide and an extraction method thereof. The extraction method comprises the following steps of: immersing spirulina which is taken as a raw material at room temperature for seven days by using industrial ethanol, pouring ethanol, airing solid, carrying out extraction for many times by using boiling water, dialyzing fluid water for two days, after concentrating intra-dialysate, centrifuging, adding a trichloroacetic acid solution to eliminate protein, neutralizing by adding an NAOH solution till the pH value is 6.5-7.5, after dialyzing and concentrating fluid water, centrifuging, adding triploid product of industrial ethanol into supernate, standing for one night, centrifugally collecting precipitation, and drying after washing through absolute ethyl alcohol and acetone respectively so as to obtain crude aqueous extraction spirulina phatensis polysaccharide; and carrying out solid residue alkali extraction to obtain crude alkali extraction spirulina phatensis polysaccharide, and separating and eluting crude polysaccharide so as to obtain pure polysaccharide. The chemical structure of the spirulina phatensis polysaccharide is characterized in that a main chain is composed of 1,4-connection alpha-D-glucose; average every eight main chain residues have one branch; substitution is at the sixth position of a branch point; and the branch is composed of one or two 1.6 connection glucose. The extraction method of the spirulina phatensis polysaccharide disclosed by the invention is simple and convenient and has good reproducibility.