Abstract: TIG arc was constricted by insulating plate covering onto side wall of square groove 4 mm in width in the narrow gap welding test.The heating characteristic of TIG arc was analyzed by various melting regions area of groove cross section and their proportion.The results indicate that the constricting action of insulating plate on arc root can restrict the arc in the bottom of square groove to intensify the heating effect on corner, and prevent the arc burned on the sidewalls.The arc length,intensity of constriction effect on arc root and welding current are matched with each other to make the sidewall roots bonded with sidewall of groove,When the arc length is too long, the arc will climb up along the sidewall directly and cannot be stopped by insulating plate.The excessive constriction effect on arc root and high welding current can result in the insulating plate melting,and the arc is instable.