Abstract: Objective J-laying is the most suitable method for laying submarine pipelines and installing Marine risers in deep water. Automatic pipeline welding equipment is often used to complete horizontal girth weld by swing welding. For problems such as asymmetrical formation and poor consistency of back formation in backing weld by swing arc P-GMAW horizontal welding, backing process method of narrow gap horizontal welding surface tension transition (STT) based on rotary arc was proposed. Methods X65 pipeline steel sheet butt narrow bevel was taken as the test object, welding process of deep water riser J-lay was simulated. Results Based on Minitab-based Taguchi method, an orthogonal test model was established. Through welding process test, influence of backing process specifications of narrow gap horizontal welding STT and different combinations of welding parameters on law of weld formation was explored. Conclusion Back formation of backing weld had good consistency without incomplete fusion, incomplete penetration and other defects.