Abstract: In this paper,the problems on compressive plastic deformation in the weld and the principle of welding stress and strain described by D.Ladayi have been discussed.It was pointed out that there is no compressive plastic deformation,but tensile plastic deformation for the weld undergoing melting solidification process.The range of tensile plastic deformation zone is much larger than that indicated by D.Ladayi.The principle diagram put forward by D.Ladayi was modified,and an new principle diagram of welding thermal stress and strain was advanced.In the new diagram,3 curves are heating up cooling down,compressive tensile and tensile elastic deformation tensile plastic deformation respectively.The temperature positioned on the isothermal line was definitely called as melting point,the range surrounded by it is welding melting pool.The concept of elastic unload was chaged into elastic deformation zone.Whole weldment includes practically elastic deformation zone,compressive plastic deformation zone,tensile plastic deformation zone and melting pool.