Abstract: AC TIG(AC:alterative current;TIG:tungsten inert-gas)welding and pulsed TIG welding were adopted to join the distorting magnesium alloy AZ31B plates. The electric arc was studied with high speed camera and the infrared thermal imaging system. The microstructures of joints were investigated and tension test was conducted. It was found that the electric arc of pulsed TIG welding was bigger than that of AC TIG welding when in the peak current. The temperature curve showed that pulsed TIG welding had a high cooling rate.The crystal grains in the heat affected zone were finer and the heat affected zone was smaller when welded with pulsed TIG welding. Tension test showed that the joint had high strength.