Abstract: A new method named ultrasonic-assisted tungsten inert gas (U-TIG) welding was invented by connecting a TIG welding torch to an ultrasonic piezoelectric transducer.The ultrasonic wave was transferred through the welding arc.The welding penetration and depth to width ratio increased,the grain size reduced and the mechanical properties were improved.However, the ultrasonic vibration system was very sensitive to the temperature.The natural frequency of ultrasonic vibration system changed as the temperature went up.The amplitude,amplified from ultrasonic horn,would decrease and even disappear.In order to ensure that the ultrasonic vibration could be amplified stably by ultrasonic horn,an annular groove for circulating water was designed in horn to cool the ultrasonic vibration system.The temperature measurement show that after cooling,the temperature in ultrasonic transducer and horn decreased from 62℃ and 53℃ to 33℃,respectively.The U-TIG welding of 8mm thick aluminum alloy sheet,without and with cooling,were carried out.The results show that the length of stable weld increased from 80 mm to almost 380 mm with cooling.The stability and carrying capacity for large current of U-TIG welding torch with cooling system was improved significantly.