Abstract: It is necessary to set up a purification stage in the middle of cascade for removing light impurities, which are formed in the process of isotope separation. These light impurities are harmful to the stable running of cascade if the impurity content is high and the cascade is long. The separation equations of a cascade were solved by a nonstationary method to investigate the feasibility of setting up a purge stage in the middle of the cascade. The effects of the purge stage, and the influence of the cut of the cascade and the purification ratio on the abundance of the light impurity and the target component were studied. The results indicated that the enrichment of the target component and the removal of the impurities could be accomplished simultaneously by a suitable choice of the cascade parameters when the purge stage was located in the middle of the cascade.