Three kinds of donor-acceptor (D-A) type semiconducting copolymers in which electron donating unit of dithienopyrrole (DTP) and accepting unit of benzothiadiazole (BT) connected with different number of thienyl spacer (x = 0-2) were synthesized and used as electron donor materials in polymer solar cells (PSCs) combined with fullerene derivatives. The optical band gaps of the polymers can be tuned by changing the number of the thiophene spacer from 1.41 eV to 1.61 eV. Electrochemical measurements showed that the increase of band gap is mainly due to the change of the lowest unoccupied molecular orbital (LUMO) energy level. The power conversion efficiency (PCE) of the polymer solar cells based on these three polymers and PC70BM reached 3.12% with x = 2 under the illumination of AM 1.5, 100 mW/cm2 This approach could provide a simple strategy for designing high-performance D-A type photovoltaic polymers based on the existing polymers and a large potential to improve their performance further.