The optical nonlinear properties of CdSeS quantum dots (QDs) are investigated by Z-scan technique using the model locked Nd∶YAG laser second harmonic radiation (λ wavelength of 532 nm, pulse width of 35 ps). The experimental results show that the quantum dots have large two-photon absorption (TPA) which originates from the third order optical nonlinearity effect. According to the absorption and fluorescence spectra, it is found that this nonlinear absorption originates from the TPA of CdSeS quantum dots. Z-scan curve of CdSeS quantum dots is measured under different incident intensities, and the experimental results show that the quantum dots have nonlinear refractive index of 1.9×10-8 esu and average TPA cross section of 25283 GM, which are two orders higher than values of commonly used ZnS QDs.