Two novel poly[(3-alkylthiophene-2,5-diyl)-(benzylidenequinomethane-2,5-diyl)s] derivatives, poly[(3-butylthiophene-2,5-diyl)-(p2N,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. 62×10-9and 1. 22×10-8ESU, 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 conjugatedπelectronics.