Abstract: The software of SYSWELD was used to simulate the welding deformation in inner pipe wall when in-service welding onto active gas pipeline and the deference of welding deformation between in-service welding and routine welding was compared.The results show that,for a point in the region near the in-service welded seam,its deformation increases when the welding heat source closes up,and achieves maximum when the welding heat source passes through this point.The deformation decreases during the cooling process.The deformation of in-service welded joint has much difference with routine welded joint.The transient deformation and residual deformation of in-service welded joint are all convex deformation,but the transient deformation of routine welded joint changes from convex deformation to concave deformation during the cooling process and the residual deformation is concave deformation.When heat input is increased,the transient deformation and residual deformation in the region near the in-service welded seam are also increasing,but the transient deformation and residual deformation in the region far from the in-service welded seam are decreasing.