The low bandgap 3-hexylthiophene/pyridine copolymers (P3HT/PY) were synthesized and the composites of P3HT/PY and CdSe QDs were prepared. The characterization are performed with 1H NMR, UV-vis, PL, TEM, XPS and cyclic voltammetry (CV). The results show that the P3HT/PY copolymers have an electrochemical bandgap of 1.67 eV, while the composites show a broader absorption and a lower bandgap than the copolymers. The highly efficient fluorescence quenching of the composites indicates that the ultrafast charge transfer happened between the P3HT/PY copolymers and the CdSe QDs. The composites show a uniform dispersion and good compatibility, and the surface chemistry study verified the chemical bonding between them. In addition, both the copolymers and the composites exhibited p- and n-type doping ability, and the P3HT/PY/CdSe composites have a better donor-acceptor match, which is a promising p-n type semiconductor material.