A new compound N-(6-hexanol)-2-methoxy-8-nitroazophenyl carbazole(HMNAC) was synthesized by electrophilic substitution reaction and post-azo-coupling reaction,and the structure of HMNAC was characterized by IR,¹H NMR and ¹³C NMR. The molecular structure of HMNAC was optimized at the B3LYP/6-31G(d) level. Single point energy was calculated at the MP2/6-31G(d) level. The relative energy E_R between cis-trans isomers of HMNAC was found to be 52.43 kJ·mol^-1. The third-order nonlinear optical properties of HMNAC were studied by Z-scan technique under nanosecond excitation at 532 nm. Closed-aperture curve has revealed that HMNAC has self-defocusing effect. Open-aperture curve indicated that both saturated absorption and reverse saturated absorption exist in HMNAC. The second-order hyperpolarizability γ of HMNAC was 2.59×10^-29 esu,and the figure of merits(FOMs) reached 1.5.