AuGe alloy has the characteristics of low contact resistance and good adhesion to GaAs substrate. It is widely used in metal/semiconductor (M/S) devices to form Ohmic contact. AuGe/Au material is deposited on GaAs surface, after rapid thermal annealing at different temperatures, the interfacial characteristics of Ohmic contact of the samples are analyzed by using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffractometer. With the increase of annealing temperature, the contact resistance shows a V-type trend. AuGe is partially decomposed. Au and Ge diffuse into the GaAs interface. Ga and As diffuse into the metal layer. The dark gray pores appear in the metal surface, and the main components are AuGa, AuGaAs etc. The bonding energy of Ge and Ga can increase 0.3～0.6 eV. The proportion of Ga3+ and Ge4+ increase, and the materials containing 10% Ge is selected as n-GaAs contact electrode. Excellent ohmic contact can be formed when the annealing temperature ranges from 380 °C to 420 °C.