The soluble π-conjugated polymer materials, poly(2-methoxy-5-butoxy)-p-phenylene vinylene (PMOBOPV), poly[2-methoxy-5-(3′-methyl)butoxy]-p-phenylene vinylene (PMOMBOPV) and poly(2-methoxy-5-octoxy)-p-phenylene vinylene (PMOCOPV), were prepared by dehydrochlorination reaction from p-methoxylphenol and alkyl bromide as raw materials. Their third-order nonlinear optical properties were studied with the backward degenerate four-wave mixing (DFWM) at 532 nm. The experimental result showed that the third-order nonlinear optical susceptibilities χ(3) values of PMOBOPV, PMOMBOPV and PMOCOPV were 3.14×10-10, 5.96×10-10 and 3.71×10-10 esu, respectively, and corresponding hyperpolarizability γ values were 4.22×10-28, 7.78×10-28 and 5.00×10-28 esu, respectively. The effects of molecular structures on nonlinear optical properties of poly(p-phenylene vinylene) (PPV) derivatives were also studied. Moreover, the optical bandgaps, Eg, of PPV derivatives were measured by spectrometer and the result of data fitting showed that Eg values of PMOBOPV, PMOMBOPV and PMOCOPV were 2.08 eV, 2.03 eV and 2.05 eV, respectively.