The invention discloses a preparation method of spirulina polysaccharide. The method comprises the following specific steps: preparing raw materials, extracting a mixture, separating an extract, carrying out cooling, carrying out purifying and precipitating, washing a precipitate, carrying out solid-liquid separation, and carrying out crushing and sieving. According to the invention, damage of spirulina to a polysaccharide structure in a high-temperature drying process is avoided, the yield and the quality of the polysaccharide are improved, the alcohol consumption required by decoloration isreduced, cost is saved, and the method is more practical. The extraction amount of spirulina polysaccharide is increased, the extracted polysaccharide is used for adjuvant therapy drugs on the market,and the effects of resisting tumors, resisting radiation, promoting DNA synthesis and enhancing immunity are achieved. The yield and the quality of the polysaccharide are improved, the alcohol consumption required by decoloration is reduced, the production cost can be greatly reduced, and the quality is improved. The polysaccharide is prepared by taking fresh spirulina as a raw material, any chemical reagent is not required to be added for treatment, chemical residues are avoided, and an experimental result is closer to a required result.