The composites of polyaniline/multi-walled carbon natotubes (PANI/MWCNT) were prepared by electrostatic interaction between MWCNT and aniline via in situ polymerization. The results of Fourier transform infrared spectroscopy, Raman spectroscopy and ultraviolet-visible absorption spectroscopy indicated that aniline molecules were adsorbed and polymerized on the surface of MWCNTs. The composite dimensions were observed by scanning electron microscope and transmission electron microscope, the diameter of PANI/MWCNT was measured to be about 40—70 nm, and the thickness of the coated polyaniline was about 15—20 nm. Third-order optical nonlinear susceptibility of PANI/MWCNT composites were measured by degenerate four-wave mixing. The results showed that the resulting PANI/MWCNT composites exhibited larger third-order optical nonlinear susceptibilities than that of pure PANI, which can be ascribed to the π-electron coupling between the PANI and the MWCNTs.