Provided are an adsorbent for trapping a radioactive iodine gas generated in a process of oxidizing a nuclear fuel at a high temperature after use and a method of preparing the same, and more particularly, a radioactive iodine gas adsorbent which is formed of bismuth as a main component, thereby exhibiting an excellent radioactive iodine gas trapping capability and an excellent thermal stability after trapping, and a method of preparing the same.An adsorbent for trapping a radioactive iodine gas prepared by a method of preparing an adsorbent for trapping a radioactive iodine gas according to the present disclosure may effectively trap a radioactive iodine off-gas generated in a nuclear fuel pre-treated oxidizing process after use.Particularly, the adsorbent may trap iodine in a larger amount, which is twice or more, than a silver-containing zeolite widely used to trap a radioactive iodine gas, and the trapped iodine forms a stable compound, which is more advantageous for long-term storage.In addition, since an iodine gas is trapped using inexpensive bismuth, instead of expensive silver, in consideration of trapping a large amount of a radioactive iodine gas, the adsorbent has very excellent economic feasibility.