Al-Cr composite coating of TC21 alloy is prepared by the combined process of multi-arc ion planting and doubleglow plasma surface alloying technology. And the high temperature oxidation behavior is studied at 750，850，950 ℃. The results show that the Al-Cr coating, consisted of pure Al layer, AlCr alloy layer, pure Cr layer and diffusion layer, is about 48μm in thick. And it is dense, with no cracks, and combined with substrate firmly. At the process of oxidation, a dense Al2O3 layer is formed on the Al-Cr coating surface, which prevents the internal diffusion of O and the external diffusion of Ti, and makes the composite coating possess an excellent protection performance. Cr promotes the formation of dense Al2O3 and improves the self-repairing function of AlCr coating.The Al-Cr composite coating significantly increases the oxidation resistance of TC21 alloy.