To prepare a novel gas sensor being able to recognize molecules, waterborne poly(β-cyclodextrin-block-polydiethylene glycol hexandioic ester) (i.e., waterborne β-CD-block-PDEA) was synthesized and filled with carbon black (CB). It was found that the composites’ electric resistance remarkably changed when the composites were exposed to the vapors of small size molecule solvents (including dichloromethane, chloroform and tetrahydrofuran, etc.). However, almost no response could be detected in the vapors of large size molecule solvents (like n-pentane, ethylbenzene and hexane). Besides, the responsiveness gradually decreased with increasing molecular size of organic solvents. The results evidenced that the composites have acquired considerable selective sensitivity towards gaseous analytes in terms of their molecular sizes. β-cyclodextrin rather than polydiethylene glycol hexandioic ester played the leading role in this aspect, which was explained from the viewpoint of host-gust chemistry. That is, the response mechanism is mainly based on the molecular discrimination behavior of the host compounds. The small analytes could be entrapped by the β-cyclodextrin cavity due to van der Waals force. On the other hand, solvents’ permittivity also played an important role. The molecules with high permittivity were difficult to enter the hydrophobic cavities. The present study demonstrated the composites could serve as candidates for gas sensors capable of molecule discrimination.