Abstract: Objective Pipeline welding in mountainous areas in China is unavoidable, and the current internal welding machine is only used for pipeline welding with a slope of less than 12°. The purpose is to verify the applicability of the newly developed internal welding machine at a slope of 25°, and provide theoretical guidance for the formulation of root welding process with internal welding machine for large-slope X70 pipeline steel. Methods X70 pipeline steel was welded by the automatic welding process with internal welding machine for the root welding and external welding machine for the hot welding, fill and cap welding. After welding, the microstructure of welded joint was observed by scanning electron microscope (SEM) and backscattered electron diffraction (EBSD). The mechanical properties of welded joint such as tensile, bending, hardness, impact and notch hammer break were tested. Results Cross-sections of the weld is consistent in the flat, vertical and vertical weld positions. The base metal is mainly composed of acicular ferrite, lath bainite. The weld and the heat-affected zone (HAZ) is mainly ferrite and bainite, but the morphology of both ferrite and bainite in different welding layers is different. The large angle grain boundary is mainly between the base metal and the weld grain boundary, and the strain is mainly concentrated at the large angle grain boundary. The tensile strength of the joint is higher than 570 MPa. There is no crack in the bending specimens. The hardness value is less than 400 HV10. The impact energy is high, and from high to low are the fusion line, heat-affected zone, weld. There are no defects such as lack of weld penetration, porosity, slag inclusion and non-fusion on the fracture surface of the notch hammer break. Conclusion The welded joint is well formed, and the mechanical properties of the welded joint can meet the standard requirements, so the newly developed internal welding machine can be used for welding 25° X70 pipeline steel.