A novel side-chain conjugated polythiophene derivative poly［3-（5′-octyl-thienylene-vinyl）-thiophene］ （POTVTh） was synthesized. The polymer films showed a broad absorption band and a narrow bandgap of about 1.82 eV. The third order nonlinear optical properties of the film were measured by Z-scan technique at 800 nm, which showed the nonlinear absoption coefficient, the nonlinear optical refractive index and third order nonlinear optical susceptibility as 5.63×10-7cm/W, -6.38×10-11cm2/W and 4.32×10-9esu, respectively. The results are one to two orders of magnitude larger than those of normal conjugated polymers without conjugated side-chain, indicating the good nonlinear optical properties of the polymer.