Abstract: Concepts of "the three criticals and the two optimums", i. e the critical operating pressure ratio (R-), selectivity coefficient (α-) and feed concentration (x1-) ,and the optimum operating pressure ratio and selectivity coefficient are first proposed. And through calculation, analysis and optimization in the range of x1≥0.4, R≤100 and α≤100, a general regression equation is obtained and shown as followsR-=2+B/(α-+A)where parameters A and B are related to x1- only, and the regression equations of xf- and A B are also obtained respectively. Moreover the following regression equations are obtained in the range of R≤1000and α≤1000R-=2+2. 15205/(α-)1.000568+2.07563 for xf-=0.95R-=2+6.2727/(α-)0.960+3.8307 for xf-=0.50