Experiment setup of femtosecond laser pump probe was established, the time resolution of time-delay setting reached 67fs. By use of femtosecond laser with width of 30fs and wavelength is 796nm the dependence of transient change of reflectivity on delayed time in GaAs was measured by pump-probe method. By calculating the change of complex index of refraction (%n), free-carrier effect, lattice-temperature and carrier recombination contributions to relaxation curve was analyzed. When the carrier density N is 1.44×1018/cm3, free-carrier contribution to refraction index %nFC is -7.33×10-4, lattice-temperature %nLT is 0.85×10-4. Based on recombination rate equation, recombination lifetime of 980ps was deduced.