The invention belongs to the technical field of nuclear safety evaluation, and relates to a measuring method and a measuring system for the retention factor of a fission product in high temperature sodium. The measuring method sequentially comprises the following steps: (1) heating sodium in a heating container to a certain temperature, and mixing the heated sodium with a fission product simulantaccording to a certain ratio; (2) allowing the fission product simulant to migrate from a sodium pool to cover gas in the heating container; and (3) calculating and chemically detecting the cover gas,and calculating the retention factor of the fission product simulant from the sodium pool to cover gas. The measuring method and the measuring system can ensure the authenticity of retention factor measurement, and well support the radioactive source term analysis process of a pool sodium-cooled fast neutron reactor.