In order to study the ohmic contact property of semiconductor device,Ti/ Pt/ 200 nm Au structure was grown on p-GaAs by magnetron sputtering,the Ti thickness is 10～50 nm and the Pt thickness is 30～60 nm. The transmission line model was adopted to calculate the contact resistivity of Ti/ Pt/ 200 nm Au structure in different thicknesses of Ti and Pt. The impact of annealing parameters on ohmic contact was studied,and the mechanism that the metal shrinks from the edge of electrode at high annealing temperature was analyzed. The results show that the lowest contact resistivity is achieved when the thickness of Ti is about 30 nm,and the contact resistivity increases with the increase of Pt thickness. Ohmic contact is sensitive to annealing temperature,and the metal of electrode shrinks from the edge when the annealing temperature is 510 ℃. 40 nm Ti/40 nm Pt/200 nm Au electrode structure can obtain better ohmic contact when semiconductor device remains 30 s at annealing temperature of 420 ℃.