The ease of separation index (ESI) that is defined by the ratio of the relative volatility between the light and middle components to that between the middle and heavy components has been adopted in the literature for the selection of optimum distillation configuration for ternary separation. The present work showed that the separation requirements (the products’ purity) have a significant impact on the economic performance of distillation systems. Then we proposed a new index, generalized ease of separation index (GESI), to include the separation requirement into consideration for the selection of the optimum ternary distillation configuration. Simple column sequences with or without heat-integration, side-rectifier, side-stripper column, and dividing wall column (DWC) were evaluated as the candidates. Triangle map of ternary mixture composition was used to show the dependence of the optimal selection on feed compositions. We showed by numerical simulation that, for a given ESI, a profound change of the partition on the triangle map occurred with the change in GESI, and on the other hand, for a given GESI, slight changes of the partition on the map could be found with the change in ESI, showing stronger dependence of the economical behaviours of the ternary distillation configurations on the proposed GESI than on the ESI. Then, the dependence of the map partition on the GESI was investigated, and we found that with the increase in GESI, the area in triangle map where DWC was the optimal configuration shrunk, while the area of direct sequence with backward heat integration (DB) expanded. We concluded that the use of the newly defined index GESI can be conductive to the optimal selection of non-azeotropic ternary distillation configuration.