Abstract: The software SYSWELD was used to simulate how the internal pressure and the welding repair time affect the radial direction deformation of pipeline for the in-service welding The results of simulation was verified by the experiment. When the in-service welding was carried out in the pipeline with an internal pressure range of 0-4 MPa, the radial direction deformation increased with the increase of the pressure. When the internal pressure was in the range of 4-6 MPa, the deformation decreased with the increase of pressure. The deformation at the internal pressure of 6MPa was almost equally as that at that one at normal pressure. When the repair welding process progressed, the amount of deformation was gradually increased. In order to predict the occurrence of burned hole, the utilization of instantaneous radial deformation is better than using the radial deformation of the internal point.