Recently the screened effect of resonance excitons absorption in GaAs has been applied to develop some kinds of optical bistability devices, therefore to measure the ultra-fast excitons screened process induced by photoexcited hot carriers is of importance not only for semiconductor physics but also for its applications. We measured the time-resolved spectrum of some GaAs samples at 5 K by means of the exciteing-probing method of picosecond spectroscopy. In our experiment the single laser pulse with duration time of 25 ps at 1.06 jim was used as exciting beam, and the probing beam was from a synchronously pumping optical parametric oscillator whose output wavelength was tuned by temperature. The main results are as following: (1) The screened time of excitons in GaAs is about 600 ps at 5 K. (2) Another transparent peak which comes from the scattering of excitons emerges at 1 ns after the excitation. (3) The transition from eleotron-hole-plasma to the excitonic state (Mott transition) has not seen to be abrupt.