Abstract: This paper describes a new method of calculating the height equialent to a theoretical plate (HETP) from the value of the total enrichment of the species to be enriched in the unsteady band (E'_e) according to the equilibrium theory of the distillation type used for enriching isotopes. From this, the quantitative relations between the total enrichment and the experimental parameters are established.The results show it is favorable for reaching a high enrichment degree for a short time if the conditions with low concentration of eluent, high linear flow rate and long adsorption band are adopted for enriching lithium isotopes and that Glueckauf's theory and Snyder's formula are also suitable for light isotopes enrichment process with simple displacement mechanism.In this connection, 0.079M CaAc₂ is chosen as the eluent, 9.9cm/min as the linear flow rate and 2.35m as the length of lithium band with the natural abundance. After the band is eluted under these chosen conditions for about 140m (17days), the quantities of lithium isotopes at the edges of the band reach up to 97.5% in ⁷Li and 15.8% in ⁶Li respectively.