Abstract: The traditional viewpoint of existing heating process and neglecting melting phenomena in welding results in the error of considering compressive plastic strain in welding. It can not interpret the fact why liquation cracks appear around the fusion line and are very short, but it may be satisfactorily explained by the high temperature strain distribution characteristic figure brought up by author. It isn' t necessary to remove the inherent strain source for the aim of relieving welding stresses, sometimes the welding stresses can also be relieved effectively in the condition of inherent strain source remained. Relieving welding stress is the process of transformation from elastic strains to plastic strains.