Impurity components decrease stage by stage in a cascade of rare earth (RE) extraction separation, and adjacent stage impurity ratio (ASIR) which is defined as the ratio of an impurity’s contents in the aqueous/organic phase of two adjacent stages can be used to evaluate the capacity of impurity removal for the two stages. On the basis of extraction equilibrium and mass balance, the ASIR in a two-component extraction separation was deducted and its simplified expressions were given for different process sections according to reasonable assumptions. The calculation simulation was then carried out to obtain the ASIR distribution in the cascade. The results showed that in both the extraction and scrubbing sections the ASIR principally increased with the decrease of the molar proportion of the impurity but along with a flat appearing in the purification zone located in the middle of the cascade. The ASIR intuitively exhibits the running status of RE extraction separation and purification, which could provide a theoretic guide for investigating the influence factors of RE extraction separation process in practical industry. Impurity components decrease stage by stage in a cascade of rare earth (RE) extraction separation, and adjacent stage impurity ratio (ASIR) which is defined as the ratio of an impurity’s contents in the aqueous / organic phase of two adjacent stages can be used to evaluate the capacity of impurity removal for the two stages. the basis of extraction equilibrium and mass balance, the ASIR in a two-component extraction separation was deducted and its simple expressions were given for different process sections according to reasonable assumptions. was then carried out to obtain the ASIR distribution in the cascade. The results showed that in both the extraction and scrubbing sections the ASIR principally increased with the decrease of the molar proportion of the impurity but along with a flat appearing in the purification zone located in the middle of the cascade. The ASIR intuitively exhibits the running status of RE extraction separation and purification, which could provide a theoretic guide for investigating the influence factors of RE extraction separation process in practical industry.