Abstract: Objective In the process of fully automatic welding of X80 steel pipes, change of preheating temperature has a certain effect on microstructure and mechanical properties of welded joints. Effects of cold conditions (−30 ℃), preheating at 80℃ and preheating at 120 ℃ on microstructure and mechanical properties of fully automatic welded joints of X80 steel pipes were systematically studied in the paper. Methods Microstructure of welded joints was observed by optical microscope (OM) and scanning electron microscope (SEM), and mechanical properties of the welds under different preheating conditions were characterized in terms of tensile tests, impact tests, bending tests and hardness tests. Results The results showed that microstructure of welded joints was mainly composed of columnar crystals and a small number of equiaxed crystals. Increase of preheating temperature decreased the size of columnar crystals in microstructure, and difference of strength and impact toughness was small. Tensile samples under different preheating temperature fractured at base material. In bending test, local cracks up to 0.6 mm appeared at preheating 80 ℃, while no cracks or other defects appeared at preheating 120 ℃. Conclusion Hardness of welded joints showed a decreasing trend with the increase of preheating temperature. In cold environment, preheating was an effective measure to obtain good overall performance of welded joints, and best performance of welded joints under the condition of preheating 120 ℃.