Aluminum/steel dissimilar alloys were joined by ultrasonic assisted laser welding-brazing technology, and the forming quality, microstructure and mechanical properties of the joints welded at different ultrasonic vibration frequencies were compared and analyzed. When the ultrasonic vibration is 0 Hz, the spreading wetting ability of liquid aluminum on the surface of stainless steel was so poor that the weld forming quality was decreased, and the biting edge and welding tumor defect were detected. Besides, some porosity defects existed in the fusion welding zone, and the IMC with the shapes of zigzag and fine acicular was found in the brazing interface. A tensile strength of 164 MPa and elongation of 5.5% were obtained, respectively. When the ultrasonic vibration frequency was 60 Hz, the weld forming quality becomes better, the IMC was flat, the thickness of IMC was only 1~3 μm, and the tensile strength and elongation after breaking were 192 MPa and 8.4%, respectively.