Graphene- cadmium sulfide (G- CdS) composites are synthesized by a solvothermal method. The composites are characterized by X-ray diffraction, transmission electron microscopy and ultraviolet visible (UVvis) absorption on their structure, size and morphology, which demonstrate that cadmium sulfide quantum dots with an average diameter of about 7 nm attached to the graphene surface. The third-order optical nonlinearities of G-CdS composites using picosecond Z-scan technique at wavelength of 532 nm is investigated, pulse width of 30 ps. The results show that G-CdS composite has a positive nonlinear refractive index and properties of saturable absorption. The third-order nonlinear susceptibility of G-CdS composite is calculated to be 4.36×10^-12 esu, nonlinear absorption coefficient to be - 6.54 × 10^-11 m/W. Comparing with cadmium sulfide quantum dots, the third- order nonlinearity of composites are improved.