The invention belongs to the technical field of polymer material and the field of biomedical engineering, in particular relates to a preparation method for a biodegradable radioactive particle used for curing cancer. The method comprises the following detailed steps: the basic material of the radioactive particle with the needed shape is prepared by using a macromolecular biodegradable material with excellent biocompatibility according to demands; the obtained basic material of the radioactive particle is put into a 0.1 to 0.4mol/l surface modified liquid for dipping and generating aminolysisand carrying out grafting reaction; purified water is used for washing the grafted basic material for removing dissociative functional groups; the obtained basic material is put into a coupling agentliquid to dip for 1 to 2 hours and then is coupled; the obtained basic material is put into the liquid to dip for 12 to 24 hours and then washed to remove the dissociative functional groups; a radioactive isotope is used for replacing the iodine element in the basic material obtained by step (4); and a decoration film is collocated into a liquid and is coated on the surface of the basic material,thus obtaining the needed product. The degradation product is innoxious and harmless, and can be discharged outside the body by metabolism. The preparation method is simple and low in cost.