Naphthalene diimide (NDI) and dithienosilole (DTS) have been well known building blocks for photovoltaic materials. Herein a solution-processable low band gap conjugated polymer, PDTS-NDI, consisting of NDI moiety and DTS unit was synthesized as a new n-type material. The polymer has two broad absorption bands in the range of 300-500 nm and 500-950 nm. Power conversion efficiencies (PCEs) of the polymer blend solar cells in the range of 0.13-0.70% were obtained under simulated AM 1.5, 100 mW/cm2 irradiation, where two polymers with conjugated side chains, PT1 and TTV2, were used as p-type material. For the combination of TTV2 and PDTS-NDI, it was further optimized to give a PCE of 1.19% by adding 1,8-diiodooctane (DIO) as solvent additive, suggesting a potential of PDTS-NDI polymer for the photovoltaic application.