The invention relates to a method and a device for modulating an electromagnetic stirring and rotating magnetic field based on a permanent magnet. In the method, a magnetic field the magnetic induction of which changes in accordance with the following equation of B=B0cos(2*pi*omega*t+phi) can be generated, wherein the magnetic field amplitude B0, the magnetic field frequency omega and the magnetic field phase position phi can be modulated independently or jointly. The device adopts the magnetic field modulation method and structurally comprises a stirrer wrapped on the periphery of a crystallizer, and the internal structure thereof is characterized in that a permanent magnet is inlaid in a toroidal core; the toroidal core is connected with a programmable stepping motor and is driven by the stepping motor to rotate; and the magnetic field generated by the stirrer always keeps the same rotating mode with the stepping motor. The invention can modulate the amplitude, the frequency and the phase position independently or jointly, realize accurately controlling the flow of molten metal so as to further well control the solidification of the molten metal, improve the quality of casting blanks and increase productivity.