For efficient and accurate speciation analysis of 129I in the nitric acid solution of spent nuclear fuel in its reprocessing process, a sequential procedure for stepwise separation of different iodine species in 3 mol/L HNO3 was proposed based on the solvent extraction using CCl4 and mesitylene. Molecular iodine (I2) was first separated by solvent extraction using CCl4, and iodide (I-) remaining in the aqueous phase was oxidized to I2 by adding NaNO2 and then extracted to mesitylene. Finally, iodate (IO3-) was reduced to I2 using NH2OH·HCl and extracted to mesitylene. The separation efficiency of 98-99% for tracer amounts of 129I2, 129I-, and 129IO3- in 3 mol/L HNO3 and less than 2% crossover among different iodine species were achieved. The extraction process and mechanism of different iodine species in CCl4 and mesitylene were investigated, and the problem of crossover of different iodine species due to the low extraction efficiency of low concentration of iodine in nitric acid solution was solved. A direct transfer of IO3- from HNO3 to the mesitylene phase without conversion to I2 was observed, which was attributed to the iodination of mesitylene in the HNO3 medium. Addition of a stable iodine species carrier and repeated extraction significantly improved the separation efficiency of iodine species, making their quantitative separation achievable. This method provides an approach for speciation analysis of 129I in the acidic spent nuclear fuel solution, enabling us to investigate and control the behavior of volatile 129I in the spent nuclear fuel reprocessing process.