The invention relates to a method for preparing transparent soap by performing irradiation treatment on illegal cooking oil. The method comprises the following steps: firstly, performing centrifugal separation on the illegal cooking oil to remove impurities, and sealing by using a sealing bottle; uniformly irradiating the sealing bottle filled with the illegal cooking oil by using cobalt source rays for 5-10 minutes with the auxiliary dosage of 1-5KGy at the room temperature; filtering the irradiated illegal cooking oil, adding sodium hydroxide ethanol solution to be saponified to obtain soap paste; adding ethanol, crane sugar, distilled water and waste soap into the soap paste to be fully stirred until the soap paste is saponified; pouring the saponified soap paste into a mould to be cured and dried to obtain the transparent soap. According to the method, the illegal cooking oil is irradiated by using the cobalt rays, a DNA sequence, a protein structure and the like in bacteria tissue in the illegal cooking oil can be destroyed to cause the death of the bacteria; meanwhile, volatile odor substances in the illegal cooking oil can be modified; the prepared transparent soap has the characteristics of shallow color, asepsis, no odor and long expiration date; the soap body has a uniform crystal structure, and is high in transparency and high in foaming capability; moreover, the harmless resource utilization of the illegal cooking oil can be realized.