The effects of the different heat treatment parameters on the microstructure and mechanical property of as-extruded Mg-4.8Zn-1.2Y-0.4Zr magnesium alloy were described based on the experimental results and analysis. The results indicate that the microstructure of alloy changed obviously during heat treatment process. After T4 solution treatment, although the alloy grain has grown obviously, the plastic of alloy has been greatly enhanced. This mainly dues to the massive fish-bone W-phase decomposed into fine granular W phase, which reduces the pinning effect of the grain boundary. After T5 heat treatment, except W-phase, there is a small amount of Mg-Zn phase precipitated in the alloy, which can improve the performance of alloy. After T6 heat treatment, the W-phase has been almost complely decomposed into Mg-Zn and Mg-Y phase. The dispersively distributed phase greatly improves the strength and plasticity, which increases the ultimate tensile strength and elongation from 311MPa and 16.89% in as-extruded alloy to 374MPa and 21.97%. At last the most suitable scheme for heat treatment of Mg-4.8Zn-1.2Y-0.4Zr magnesium alloy is( 500℃×2h 200℃×48h).