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Mg-8Gd-3Y-0.5Zr(GW83K) alloys were prepared by means of the atmosphere protection method and then chamber die cast into tensile samples with cold mould. The microstructures and mechanical properties of alloys in die-cast state and different heat treatment states were investigated by optical microscopy, scanning electron microscopy and mechanical property testing. The results show that the mechanical properties of the cold-mould die-cast GW83K alloys after heat treatment are improved. In particular, the die-cast GW83K alloy after short-time low-temperature solid solution treatment (T4) has a little variation in grain size and more uniform microstructure, lamellar eutectic disappears and the second phases distribute in the grain boundary in discontinuous rod shape or granule shape. Due to microstructure improvement of GW83K-T4 alloy, the mechanical properties of the alloy at room temperature are improved such as: σb=261.7 MPa, σs=240.8 MPa and δs=6.0%, which are increased by 21%, 28.4% and 30.4% compared with those of die-cast GW83K alloy respectively, and GW83K-T4 alloy has better elevated temperature mechanical properties.