A new method was presented from the view of mechanics to prevent welding hot cracking for aluminum alloy by employing transverse ultrasonic during welding. The mechanical condition of welding hot cracking was discussed. The material-mechanical properties matching model within brittle temperature region (BTR) metals was established and the plastic flow and the variation of the strain field in welds by transverse ultrasonic were analyzed numerically. The simulation results show that the transverse ultrasonic impact produces transverse compressive plastic strain in BTR metal, and the plastic deformation is transferred to the weld centre through the metal within BTR. With the ultrasonic load amplitude increasing, the compressive stress in weld centre increases. Experiment was carried out through the ultrasonic extrusion in BTR area. It is indicated that the simulation results and the experiment results are in good accordance, which proves the reliability of the new method