The full-scale simulation experiment of the main pipeline installation is the most important work before the main installation and welding of the main pipeline of the nuclear power plant,and the backing welding is the first process of welding. The quality is the basis for ensuring the weld molding and performance of the main pipeline. As a consequence,controlling the quality of the backing welding is the key to the success of the full-scale simulation experiment of the main pipeline. This paper is mainly aimed at the burnthrough,concave and non-welding defects occurred in the three-weld backing welding process of the main pipe of a nuclear power plant. It is analyzed from five aspects:human,machine,material,method and ring. And combined with the test verification after the cause analysis,find out the shortcomings in the simulation of the trial,make clear the quality control measures of the main pipe backing welding and provide some guidance for the follow-up of the main pipeline work.