The ultraviolet optical transition of the isomeric state in Th-229 has attracted much attention recently due to its potential application to building an atomic/nuclear clock with ultra-high precision. However, the lowest nuclear excitation energy and the lifetime of the first excited state of Th-229 were not measured directly and precisely until now, and how to precisely determine this isomer state of the Th-229 is an urgent requirement. Here an experimental approach of using a technique similar to that of dielectronic recombination to measure the transition energy of the isomer state of Th-229 at heavy ion storage rings is described. It is expected that the resonant transition can be found and determined with a precision better than several milli-eV.