A novel side-chain conjugated polythiophene copolymer with a narrow band gap of 1.72eV, poly -thiophene (POTVTh-Th), is synthesized by the Stille coupling reaction. The third order nonlinear optical properties of the copolymer are measured by femtosecond Z-scan technique at 800nm. The results indicate that the third order nonlinear susceptibilities of the polymer solution and the film are 8.84×10-10esu and 7.25×10-9esu, respectively, which are 2.17 and 1.68 times bigger than those of its homopolymer, showing enhanced third order nonlinear optical properties and the good nonlinear optical properties of this side-chain conjugated copolymers.