The optical nonlinear absorption and nonlinear refraction properties of CdTe and CdS quantum dots(QDs) were investigated by Top-hat Z-scan technique. The experimental condition of Top-hat Z-scan was at a wavelength of 532 nm and a pulse width of 190 fs. Under the the action of femtosecond laser pulses, the experimental results show that the nonlinear absorption of CdTe QDs is saturable absorption and the nonlinear absorption of CdS QDs behave as reverse saturable absorption. The nonlinear refraction of CdTe QDs self-defocusing, and CdS QDs show self-focusing. The nonlinear absorption coefficients of CdTe QDs and CdS QDs with dimensions of 2.6 nm and 2.4 nm are -9.26×10-14 m/W, 0.78×10-14 m/W, respectively, and the nonlinear refractive index coefficients are -0.86×10-20 m2/W, 1.46×10-20 m2/W, respectively, and the third-order nonlinear polarizabilities are 2.72×10-15 esu and 1.36×10-15 esu, respectively. It was shown that the optical nonlinear absorption and nonlinear refraction of cadmium semiconductor QDs with different materials under the similar sizes are different. Based on the experimental results, the mechanism was analyzed.