Abstract: The semi-solid A356 alloy billet is prepared by low-temperature pouring and gain refining. This paper studies the microstructure evolution rule effected by RE refiner during isothermal heating treatment. Results show that the addition of RE refiners obvious improves A356 alloy's microstructure of pre -isothermal casting heat treatment and post heat-treatment. The optimizing isothermal heating technology condition for the semi-solid nondendritic A356 alloy billet, under the co-action of refining and low temperature casting, is 30min 's insulation at 583 ℃. The casting has good thixotropy performance, the average equal -area -circle grain diameter and the average grain roundness being80μm and0.83respectively.