The research of THz Quantum Cascade Lasers(QCL) includes basic theory and applications，which shows great scientific and applicable values. It is the hotspot in THz fields and has been paid attention to by many countries. To prepare THz QCL，GaAs quantum well superlattice needs to be obtained firstly. During the preparation of GaAs quantum well superlattice，the problem of As absence needs to be solved. In this paper，GaAs has been prepared by Laser Molecular Beam Epitaxy(LMBE) and the corresponding results have been studied. RHEED indicates that GaAs can grow layer by layer. The results of situ XPS，UPS et al，show that there exists the absence of As during the growth of GaAs，and the laser energy has obvious influence on the content of GaAs. To obtain the ideal ratio of Ga:As，a high laser energy of 600 mJ is needed.