Abstract: Based on the equilibrium plate model of the distillation type,a set of theoretical equations has been deduced for the isotope's stationary band,and a new method of calculating the height equivalent to a theoretical plate (HETP) used for isotope enrichment was suggested.Moreover,the quantitative relations between the total enrichment and the experimental parameters (concentration of eluant,flow rate,length of band and displaced distance) were established.The expression of enrichment variation,while approaching the stationary state,was corrected and demonstrated experimentally.By the use of that expression,the semicontinuous isotope enrichment process was successfully estimated.