With the numerous applications of high-pressure pipeline, it is very important to simulate the in-service welding of the pipeline.The flow state of the medium in the pipeline can be analyzed, the temperature field and the stress field also can be simulated by applying sysweld software. The medium flow can reduce the welding residual stress and reduce welding stress range during in-service welding. Meanwhile, with the increase of flow rate of the medium, the maximum temperature of inner wall drops and the maximum weld residual stress increases by a slow growth. At the same time, the highest temperature of pipe wall increases rapidly with the increase of heat input, whereas the maximum residual stress of welded joint decreases by a slow trend.