A novel basic polymerized ionic liquid (BPIL): polymeric 1-[(4-ethenylphenyl)methyl]-3-propylimidazolium imidazolide was synthesized and characterized by Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR) and electron spray ionization mass spectrometry (ESI-MS). The BPIL was used as an efficient catalyst for aqueous Knoevenagel condensations with extended substrates. In comparison with common base catalysts, the BPIL showed high catalytic activity, which was ascribed to the cooperation between the strong basicity and the high surface activity. Moreover, the BPIL with high molecular weight has high surface activity and low catalytic activity. In addition, the BPIL was easily recovered and maintained high catalytic activity after five cycles of use in the system using benzaldehyde and malononitrile as substrates.