In order to improve the solution processability of 4,7-bis(thiophen-2-yl) benzo[c][1,2,5] thiadiazole (DTBT)-based polymers, novel donor-acceptor polymer PTOBDTDTBT containing DTBT and benzo[1,2-b: 4,5-b']dithiophene (BDT) with conjugated side chain is designed and synthe-sized with narrow band gap 1.67 eV and low lying HOMO energy level -5.4 eV. The blend film of PTOBDTDTBT and PC 71 BM exhibits uniform and smooth film with root-mean-square (RMS) surface roughness 1.15 nm because of the excellent solubility of PTOBDTDTBT when six octyloxy side chains are introduced. The hole mobility of the blend film is measured to be 4.4 x 10(-5) cm(2) V-1 s(-1) by the space-charge-limited current (SCLC) model. The optimized polymer solar cells (PSCs) based on PTOBDTDTBT/PC71BM exhibits an improved PCE of 6.21% with V-oc = 0.80 V, J(sc) = 11.94 mA cm(-2) and FF = 65.10%, one of the highest PCE in DTBT containing polymers.