Two novel poly[(3-alkylthiophene-2,5-diyl)- (benzylidenequinomethane-2,5-diyl)s] derivatives, poly[(3-butylthiophene-2,5-diyl)-(p-N,N-dimethylamino)benzylidenequinomethane-2,5-diyl)](PBTDMABQ) and poly[(3-octylthiophene2,5-diyl) -(p-N,N-dimethylamino)benzylidenequinomethane-2,5-diyl)](POTDMABQ), were synthesized. The band gaps of the two polymers are calculated as 1.75eV for PBTDMABQ and 1.69eV for POTDMABQ, respectively. The homogenous films of the two polymers were prepared and their third-order nonlinear optical properties were studied by the backward degenerate four-wave mixing at 532nm. By using the relative measurement technique, the third-order nonlinear optical susceptibilities of PBTDMABQ and POTDMABQ are calculated as 5.62×10~(-9) and 1.22×10~(-8)ESU, respectively. It is found that substituted alky groups have strong effects on the band gap and nonlinear optical properties of the two polymers. The relatively big third-order nonlinear optical susceptibilities and small band gap of POTDMABQ resulted mainly from the longer alkyl with strong electron-donating ability can enhance the delocation degree of conjugated π electronics. Two novel poly [(3-alkylthiophene-2,5-diyl) -benzylidenequinomethane-2,5-diyls] derivatives, poly [ benzylidenequinomethane-2,5-diyl] (PBTDMABQ) and poly [(3-octylthiophene2,5-diyl) - (pN, N-dimethylamino) benzylidenequinomethane-2,5-diyl] gaps of the two polymers are calculated as 1.75  eV for PBTDMABQ and 1.69  eV for POTDMABQ, respectively. The homogenous films of the two polymers were prepared and their third-order nonlinear optical properties were studied by the backward degenerate four-wave mixing at 532  nm. By using the relative measurement technique, the third-order nonlinear optical susceptibilities of PBTDMABQ and POTDMABQ are calculated as 5.62 × 10 -9 and 1.22 × 10 -8 ESU, respectively. It is found that substituted alky groups have strong effects on the band gap and nonlinear optical properties of the two polymers. The relatively big third-order nonlinear optical susceptibilities a nd small band gap of POTDMABQ derived mainly from the longer alkyl with strong electron-donating ability can enhance the delocation degree of conjugated π electronics.