Abstract: Based on thermal-elastic-plastic theory,a 3-D finite element model of root butt welding of SA335 P91 steel pipe was carried out with ANSYS software.Using moving heat source,the transient temperature field and welding stress-strain field in root butt welded joint of a pipe were analyzed.Moreover,the distributions of axial and hoop transient stresses to close the two-half seam joint were investigated.The calculation results show that the axial and hoop stresses at the keyhole center are tensile,specially,the edge stress effect is remarkable where the hoop stresses are stronger than the axial ones,and how the holding time of heat source in the keyhole and the welding current influences the distribution of stresses and strain during the high temperature solidification is also discussed.