Research conclusions:Based on the theoretical analysis and according to the actual production process，the negative factors were overcome by determining the reasonable support location of roller and properly configuring weight equalizer．Based on it，a method to do girth welding of polygonal line tube with submerged－arc welding was found．With this method，the 100% of automatic welding of girth joint in factory workshop can be realized，which is up to 90% of the full－bridge tube girth joint with features of less welding defects，stable welding quality and improving welding efficiency．