Abstract: Extraction is a promising method of lithium isotopes separation, which is expected to replace the mercury method to eliminate mercury harm. The separations of lithium isotopes by solvent extraction system mainly use the difference of partition coefficients of ⁶Li and ⁷Li in two-phase solvents to separate lithium isotopes. There are several solvent extraction systems to separate lithium isotopes, mainly ethers, and crown (hole) ether extraction systems have the best separation effect whose coefficient could reach to 1.081. The solvent extraction systems for lithium isotope separation need to meet the following requirements: stable system with appropriate extraction partition coefficient for lithium; existence of obvious lithium isotope separation effect; high speed of two-phase flow contact and isotope exchange. Consequently, solvent extraction is a promising method for the separation of lithium isotopes. In order to obtain high abundance isotopes, multi-stage extraction is needed in the extraction process. Using centrifugal extractor instead of extraction clarifier to form an extraction cascade system can greatly improve the separation efficiency. Based on the principle of extraction, centrifugal extraction and cascade theory, the concept of split ratio was introduced, and the single-stage and multi-stage mathematical models and equilibrium time models of centrifugal extraction cascade separation of lithium isotopes were established. This paper is aim at isotope exchange section, which is the critical part of centrifugal extraction cascade. In the isotope exchange section, the water phase and organic phase are continuous flow in each stage. Theoretically, the ratio of organic phase to water phase and the content of lithium in each phase are exactly the same. After isotope exchanging and reclaiming, the two phases were inversed by extraction and stripping and return to the isotope exchange section. Thus, cascade’s split ratio of all stages is 0.5. In addition, the equilibrium time of cascade entirely depends on isotope exchange section which has the longest equilibrium time. The equilibrium time of isotope exchange section can be estimated by analyzing the transport rate of enriched component. Centrifugal extraction cascade is a kind of reflux rectangular cascade. The amount of feed has a great influence on the cascade stages. There is a limit of maximum feed abundance in the cascade. The equilibrium time of the cascade is affected by the target abundance and the residence time which is relied on centrifugal extractor’s processing capacity. The multi-step cascade can effectively reduce the equilibrium time. The mathematical model can guide the process design and provide a theoretical basis for the next industrial application.