Abstract: It is derived that the varying manner of instantaneous constant composition region with batch distillation of a binary ideal mixture under constant reflux ratio and ideal operating condition is determined with the numeric of f(x_F)［Eq.(15)］ by setting up the function f(z)［Eq.(12)］,and its algorithm of minimum reflux ratio can be improved.This paper analyzed the cause leading to high energy consumption of batch distillation under constant reflux ratio to meet the demand for a high light distillate composition and high average concentration of light component in the overhead product as compared to the case of operation with a constant distillate composition.