Abstract: The X70 pipeline steel high temperature tensile test and simulation that based on the welding software of SYSWELD were used to study the in service welding residual strength size effect. The results of tensile test showed that, when the temperature was 700℃, the strength of X70 pipeline steel was 250 MPa, when the temperature rised from 800℃ to 1 300℃, the strength reduced nonlinearly from 102 MPa to 19 MPa. The equivalent method of defect, the numerical simulation results and the high temperature mechanical properties research data were combined to establish residual strength burn through criterion of in service welding, and the influence of the pool size effect to the criterion was also be studied, when the area of the equivalent defect increased to 1.1 times the bearing capacity decreased from 3.38 MPa to 0.5 MPa, which showed that the bearing capacity was decreased significantly with a slightly increase of molten pool area, the pool length had little effect on the residual strength, the pool depth and pool width have significant effect on the residual strength, and the change of penetration has a greater effect on the strength distribution of the high temperature area, the pool size effect is obvious.