多元同位素分离理论是稳定同位素分离的理论基础。矩形级联具有机器种类少，连接简单等特点，适合分离种类多，小规模生产的稳定同位素。用矩形级联分离多元同位素，在外界参数不变的情况下，改变供料位置，目标同位素的产品丰度有较大差别，这就要求选择最佳供料位置使产品丰度最高。从推导二元小分离系数短矩形级联的最佳供料位置出发，分析了当分离系数增大及级联加长时供料位置的变化趋势，并通过求解非定常态多元分离级联丰度非线性方程组，得出多元分离的最佳供料位置的变化趋势及最佳供料级入口丰度和供料丰度之间的关系 Multivariate isotope separation theory is the theoretical basis for stable isotope separation. Rectangular cascade with a small number of machines, simple connection, suitable for the separation of many types of small-scale production of stable isotopes. Rectangular cascade is used to separate multivariate isotopes. When the external parameters remain the same, the feed position is changed, and the product abundance of the target isotope is greatly different. This requires that the optimal feed position be selected to maximize the product abundance. Starting from the optimal feed position of short rectangular cascade with small coefficient of binary separation, the change trend of feed position when the separation factor is increased and the cascade is increased is analyzed. By solving the unsteady state multiple elemental cascade abundance Nonlinear equations, the trend of the optimal feed position of multiple separation and the relationship between the best feed level inlet abundance and feed abundance