Abstract: Considering the structure characteristics of small-diameter thick-walled pipe and the properties of 40CrMnMo steel,narrow gap pulsed MAG welding technique was preferred to weld the oil drill collars. A large number of experiments were conducted to improve the gas protection,arc morphology and stability, and weld appearance to avoid the sidewall fusion defects during welding,and the narrow gap multi-layer welding parameters were optimized. Furthermore,a new special narrow gap welding torch was designed,which had advantages such as simple and compact structure,good accessibility,good gas protection, easy ignition,stable arc combustion and free from sidewall ignition. The mechanical tests and microstructure analysis show that the mechanical properties of joints can satisfy the practical requirements of the oil drill collars.