Six new organic conjugated azo compounds were synthesized and their structures were confirmed by Fourier transform infrared spectroscopy(FT-IR), 1H nuclear magnetic resonance apparatus ′H NMR and elemental analysis. By using femtosecond laser, the off-resonant third-order optical nonlinearities of the compounds were measured with degenerate four-wave mixing (DFWM) technique. The third-order nonlinear optical (NLO) susceptibilities χ(3) were (3.31～3.96)×10-13 esu. The nonlinear refractive indexes n2 were (6.08～7.27)×10-12 esu. The second-order hyperpolarizabilities γ of the molecules were (3.44～4.11)×10-31 esu. The response times τ were 102～111 fs. The influence of the molecular structure on the third-order optical nonlinearity was studied. The third-order optical nonlinearity is improved by many factors such as aromatic heterocycle with small delocalization energy, long conjugate chain, formation of acceptor-donor srtucture, strong electron-donating abilities of substituents, and good coplanarity.