Abstract: Welding process is a rapid and quite non-uniform physicochemical metallurgy process,and molten pool is always one of the important areas for welding simulation.According to the principle of conservation of energy and characteristics of tungsten inert gas,(TIG) welding,the numerical model of non-steady TIG welding pool shape under moving arc was established.The heat enthalpy conception and the heat model of surfaced bielliptic distribution were introduced and the demands of welding numerical simulation was primely satisfied.The TIG welding of thick-wall pure copper adopting Ar+N₂ without preheating was studied.The comparison between the experiment value and the calculation with different process parameters showed that the model and practical course match well,and the reliability and correctness of the model are proved.