The invention relates to a preparation method of a vitrification substrate for radioactive nuclear wastes, which is characterized by comprising the following steps: mainly taking (NH4) H2PO4, Fe2O3 and H3BO3 as raw materials, proportioning, mixing, melting for 2-3.5 hours at the temperature of 1100-1250 DEG C, forming, and annealing, thereby preparing a vitrification body with glass of an iron-boron-phosphorus system as the substrate. The invention combines the advantages of high irradiation stability of the vitrification body of the borosilicate system and large waste-loading capacity of thevitrification body of the iron phosphate system. The prepared vitrification body has the advantages of excellent chemical stability and good thermal stability. Moreover, the invention has the advantages of simple preparation process and easy engineering, and can be widely applied in the curing treatment of radioactive nuclear wastes.