Abstract: According to traditional viewpoint, for the mechanical stress relieving method, the low temperature stress relieving method, the rolling stress relieving method and Low-Stress Non-Distortion Welding Technology, the residual stress relieving principle is to compensate the compressive plastic strain in the weld with tensile plastic strain which is produced in the welding process. In this paper, a new stress and strain developing process is given. The weld metal always bears tensile stress and there is no compressive plastic strain being produced in the welding process. In the brittle temperature zone, the maximum tensile stress appears in the fusing line and it decreases sharply far from the fusing line. The stress-relief principle is to reduce or eliminate elastic strain with plastic strain. In general case, the principle is to reduce or eliminate the original tensile elastic strain with tensile plastic strain. In this paper, the principle of all kinds of residual stress relieving methods is also discussed.It also is satisfactorily interpreted why the transverse liquation crack appears in the adjacency to fusing line and it is very short.