Through physical simulation, we study the influence mechanism of electromagnetic field on the solidification structure of homogeneous Zn-Bi monotectic alloy, the transformation rule of the diameter of molten secondary phase under the compound field, and the process of the shattering of the layered structure into matrix by the Lorentz force. The results show that the diameter of the secondary drop decreases with AC current density and magnetic field increasing. The influence of AC frequency has an optimal value from which the deviation can weaken the suppression of diameter, which accords with the experimental result. The layered metal liquid can be uniformly mixed under the action of electromagnetio field. The above effects can effectively release segregation of the second phase of monotectic alloy. The simulation results show the main factors of preparing the homogeneous monotectic alloy in an electromagnetic field intuitively.