Abstract: In recent years, martensitic heat-resistant steel (A335-P91) imported from the United States and Japan was applied to the main steam pipelines in the construction of large-scale power station units in China. The steel has high strength at high temperature, oxidation resistance and creep resistance with a relatively high thermal conductivity and low thermal expansion and so on. A335-P91 pipelines of 7 600 inches or so in length were used in the syngas purification device for a coal chemical group’s coal indirect liquefaction project. The A335-P91 pipelines with design pressure of 13.5 MPa and design temperature of 545 ℃ have the largest specifications for ϕ610×46 mm. According to its weldability, the welding process of A335-P91 steel pipeline was determined by taking GTAW with filler metal as backing welding, and SAW as filling welding and cover welding. The welding task was accomplished with high quality and efficiency.