Abstract: Based on Comsol Multiphysics software, an electromagnetic ultrasonic-assisted brazing model was established to study the spreading behavior of solder Sn-9Zn on the surface of SiC ceramics under the action of Lorentz force. It spreads out, accompanied by the vibration phenomenon on the surface of the solder, and then part of the solder under the coil breaks and splashes, and the solder shrinks in the center. When the coil is fed with an alternating current, an alternating alternating current with a periodic change in direction is induced around the coil. Magnetic field. The direction of the Lorentz force inside the solder points to the outside of the solder as a whole, and gradually points to the surface of the base metal at the edge of the solder. The horizontal and vertical Lorentz forces of the solder under the coil are higher than other parts, and the highest respectively are 63.96 and 31.2 kN/m³, the top brazing filler metal promotes brazing filler metal spreading under the combined action of Lorentz force, pressure difference and "collapse effect".