The pump-probe technique is an effective method to investigate ultrafast dynamics. And it is widely used in fundamental and application fields of Physics, Chemistry and Biology. The dynamics of bulk GaAs was investigated by femtosecond laser. By changing the area of pump spot, different laser fluences were obtained to excite electron from valence states to conduction states. And it was found that the amplitude of reflectivity change is different. When the carrier densityNis 1.44×1018/cm3, the change of refraction index is about DnC=-3.33×10-5. And whenNis 0.36×1018/cm3, the change is -2.0×10-5.