Abstract: When the heating pipeline is repaired with water, the state of the medium in pipeline is an important factor affecting the repair effect.By building a test device that can adjust the medium pressure and flow state in pipeline, the active TIG welding was used instead of the traditional welding method for cover welding of the linear crack on 6 mm thick pipeline, and the influence of the medium pressure and flow state on the welding effect was studied. The results show that when the medium pressure is high, the formation of molten pool is affected by the medium pressure, which can lead to secondary leakage; when the medium pressure is low, the fusion depth of the crack increases with the decrease of the pressure.Under the normal pressure with water, cracks on the pipeline can be penetrated. Compared with the electrode arc welding for covering, the crack fusion depth is greatly improved. Under the hydrostatic pressure state, the crack fusion depth is bigger than that under the hydrostatic pressure state. Under the state of hydrodynamic pressure, the crack fusion depth increases with the decrease of the medium pressure.