The performance and structure of Ni/Au/Ni/Au contact of p electrode of AlGaN semiconductor are studied. Before annealing, the p electrode has obvious rectifying characteristics. After being annealed in air at 550℃ for 3 min and then being annealed in N2 at 750℃ for 30 s, a good ohmic contact is obtained. To uncover the ohmic contact forming mechanism of this structure, the Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Energy Dispersive Spectrometer (EDS) and X-ray Photoelectron Spectroscopy (XPS) are used to study the microstructure evolution of metal-semiconductor interface. The results show that the metal schemes change sharply and an interdiffusion and interface reaction happens during annealing. The as-deposited four-layer electrode is missing and is replaced by a single metal layer. A coherent or semi-coherent relationship is established at the interface. Thus, Ga diffuses to the metal electrode and Au and Ni diffuse into the upper layer of the semiconductor at the interface. Ga, Au and Ni are enriched across the interface. Ni has an outward diffusion and reacts with O. Au has an inward diffusion to the p-Ga surface. All of these phenomena are critical to the formation of the ohmic contact.