The invention discloses a preparation method and a device of semi-solid slurry. Three processes of air current stirring, electromagnetic vibration and addition of heterogeneous nucleating agents are comprehensively utilized by the method. A graphite head with holes is connected with a hollow stirring rod, and a rotating motor drives the stirring rod to rotate through gear transmission and drives the graphite head to rotate; gas passes through a sleeve barrel and reaches the graphite head through the hollow stirring rod, and then, the gas overflows out of small holes, thereby realizing the purposes of blowing and stirring the molten metal in a rotating way; an electromagnetic vibration table can realize electromagnetic vibration on the molten metal in a crucible; the heterogeneous nucleating agents are added into the molten metal to promote further spheroidization and refinement of slurry tissues; after the molten metal is smelted, the graphite head is inserted into the molten metal for blowing and stirring the molten metal in a rotating way; and simultaneously, the molten metal is vibrated, and the heterogeneous nucleating agents are added through a powder feeding pipe, thereby preparing the semi-solid slurry with round and smooth primary grains. The prepared semi-solid slurry has the advantages of purity, no pollution, simple used equipment, easy control of process parameters, short pulping processes, small energy consumption and low cost.