The present invention relates to a method and a system for preparing radioactive elemental iodine through an isotope exchange method. According to the method, gas containing stable <127>I2 with a certain concentration is introduced into a radioactive KI3(127+131) solution, and <127>I and <131>I are subjected to isotope replacement to obtain elemental <131>I2. The system comprises radioactive iodine preparation and test branch path and a titration bypass, wherein the radioactive iodine preparation and test branch path and the titration bypass are switched through a three-way valve. With the method and the system of the present invention, the radioactive elemental iodine can be prepared quickly and safely, and the elemental iodine retention performance of the iodine adsorption device using active carbon at the high temperature can be evaluated.