Abstract: For ensuring the welding workmanship and reliability of thin-wall cylindrical pressure vessel of T250 cobalt free maraging steel, GTAW welding experiments were carried out with a thin-wall cylindrical sample and a corner weld sample. The results indicated that T250 steel has good weldability, cold cracking in GTAW joint of the thin-wall part little occurs, and neither pre-heat before welding nor heat treatment for removing stress after welding are required. In welding process, the welding current should be controlled strictly to prevent the weld defects. Besides, the strength of welded joint is not high after welding, so it is necessary to improve the joint strength to reach the similar strength of ultra-strength steel by ageing or quench annealing + aging treatment.