The invention discloses a preparation method of iron oxide hollow-sphere slurry by taking water-soluble polyester as a gel system and belongs to the field of preparation of slurry. The slurry is prepared by taking the water-soluble polyester and hollow iron oxide as raw materials; the hollow iron oxide is added into the water-soluble polyester so that the linear shrinking percentage of the formed slurry is reduced and the porosity is increased; the water-soluble co-polyester slurry has biodegradability so that the environmental pollution is reduced; and the prepared slurry has an excellent sizing performance on dacron, good wetting property and permeability on yarns, good compatibility with other slurry and good chemical stability, and is convenient to store and transport.