The present invention relates to the technical field of nanometer selenium hydrosols, particularly to a nanometer selenium hydrosol having antitumor activity, a preparation method, a preservation method and applications thereof, wherein the hydrosol at least contains nanometer selenium with a concentration of 0.5-5.0 mmol / L, and Coriolus versicolor polysaccharide protein with a concentration of 100.0-600.0 mg / L. The preparation method at least comprises: S01, adding a solution containing selenium ions and / or selenous ions to a Coriolus versicolor polysaccharide protein aqueous solution; S02, adding a reducing agent solution to the mixed solution obtained in the step S01 in a dropwise manner, and shaking; and S03, carrying out volume metering on the product obtained after the reducing reaction, and carrying out dialysis treatment. According to the present invention, the hydrosol is stable at a temperature of 2-10 DEG C; and the Coriolus versicolor polysaccharide protein has the hydrophilic hydroxyl and the hydrophilic amino, and the two groups can enhance the affinity between the nanometer selenium and the cancer cells, and can improve the uptake on the nanometer selenium by tumor cells so as to achieve the whole treatment effect of medication dosage reducing, treatment effect improving and less toxic-side effect, such that the effective scheme is provided for the combination chemotherapy of cancers in the clinic.