Two low-band-gap conjugated polymers, PSiFDCN and PSiFDTA, which consist of alternating silafluorene and triphenylamine backbone and different pendant acceptor groups (malononitrile and 1,3-diethyl-2-thiobarbituric acid) with styrylthiophene as pi-bridge, were synthesized and characterized. By changing the acceptor groups in side chain, the energy levels, absorption spectra, and band gaps of the resulted polymers were effectively tuned. As the strength of the acceptors increases, the band gap reduces from 1.83 eV for PSiFDCN to 1.74 eV for PSiFDTA. Bulk heterojunction solar cells with these polymers as electron donor and (6,6)-phenyl-C-71-butyric acid methyl ester (PC71BM) as electron acceptor exhibit high V-oc (> 0.85 V) and power conversion efficiency (PCE) of 2.50% and 3.15% for PSiFDCN and PSiFDTA, respectively.