A infinity/infinity analysis method was developed to evaluate the effect of feasibility analysis methods on designing the reactive distillation (RD) process. Its mathematic derivation revealed that results from the reactive flash-cascade model were the same as results from the infinity/infinity analysis method only if the predicted column discharge was a pure compound or a common azeotrope under chemical equilibrium, but those two results were different when the predicted column discharge was a reactive azeotrope. The same phenomenon also appeared in comparison between the singular point equation method and the reactive flash-cascade model rebuilt by a single mole-based parameter. In the case of the reactive azeotrope appearing in the column discharge, both the theoretical discovery and the case study suggested that the separating degree of a RD column predicted by the feasibility analysis was finite, and it may be broken through in reality by means of imposing greater separating power, such as more theoretical trays or a bigger reflux ratio. Results from the feasibility analysis should be checked carefully because appropriate operation parameters can change the predicted separating performance of the RD column.