Abstract: The current states of joining metal sheets by ultrasonic spot welding with numerous critical issues,such as interfacial temperature,macrostructure,microstructure,mechanical properties,general process parameter optimization,strengthening method of mechanical properties and joining mechanism were systematically summarized. The current problems were analyzed,and it was noted that the stress and strain field of welding interface,the process and mechanism of plastic deformation should be investigated in further. Influences on the quality of ultrasonic spot welded joints were processing parameters and materials properties. Processing parameters were composed of clamping force,amplitude,welding time/welding energy,power,dimensions and shape of the knurl pattern of the anvil and sonotrode tip,and so on. Materials properties included materials categories,hardness,surface roughness,degree of oxidation,and so on. The best welding process conditions could be obtained only by good coupling between process parameters and materials can. In addition,bond density and post-welded thickness were two key factors to the mechanical strength of ultrasonic spot welded joints,but a contradictory relation existed between the two key factors,how to resolve the contradictory relation was critical to enhance the joint strength.