A new algorithm, the classified enumeration, is proposed to solve computer aided molecular design of solvents for separation processes. In this method, all groups are sorted into two parts, skeleton groups and function groups. The former can constitute alkane, arene (aromatic hydrocarbon) or cycloalkane, the latter are the remainder. The skeleton groups are combined to form molecules of alkane, arene or cycloalkane as the base by enumeration. The function groups replace the corresponding skeleton groups in the molecule of the base to build other molecules. The properties of all acquired molecules are calculated using group contribution methods to find the proper solvents for the separation process. When considering industrial practicability, the number of kinds of function groups are limited so that combinatorial explosion is avoided. Finally, several examples are presented to illustrate the availability of the method.