Semi-solid magnesium alloy slurry was produced by a newly self-developed mechanical stirring machine in order to prepare continuous semi-solid slurry. The changing principle of semi-solid microstructure was studied under different conditions—the changing of temperature, stirring rate, cooling rate and stirring time. It was showed that the lower the temperature of slurry, the higher the stirring rate, the longer the stirring time and the faster the cooling rate, the grain will be smaller and more evenly rounded. When the stirring speed and temperature are the same, better semi-solid spherical grains can be gained by increasing the stirring time. At the same stirring temperature and higher stirring speed, semi-solid spherical particles are smaller, rounder and more evenly; at a low stirring speed, we can't get ideal semi-solid spherical grains even if we increase the stirring time. The ideal technological parameters of semi-solid AZ61 alloy produced with no-dendritic structure were recommended to be 600°C～605°C (stirring end temperature) 、3min～ 5min(stirring time) and 500r/min(stirring rate). Compared with conventional casting alloy, semi-solid alloy mechanical properties were greatly improved. The non-dendrite semi-solid forming mechanism was studied; mechanical stirring semi-solid non-dendritic formation was the result of a variety of factors, the key cause is mechanical shear stress.