Abstract: The article measured the critical fracture stress of E550 steel under three welding process conditions of preheating 50 ℃, preheating 100 ℃, and preheating 150 ℃ using the pin test method. The morphology of the pin fracture and the microstructure near the fracture were observed, and the sensitivity of the steel to welding cold cracks was comprehensively analyzed. The results indicate that E550 steel is sensitive to welding cold cracks, and preheating can significantly improve the steel's resistance to cold cracks. In the welding state, the HAZ coarse grain zone structure of E550 steel welding joint is Flat noodles martensite and lower bainite. With the increase of preheating temperature, the proportion of Flat noodles martensite structure decreases, and the proportion of lower bainite structure increases, which increases the resistance to cracks. Under three preheating conditions, the final fracture zone exhibits a small dimple morphology, and with the increase of preheating temperature, the proportion of dimple shaped fracture surface on the entire fracture surface increases, and the dimple size becomes larger and deeper, indicating that the plasticity of the material has been improved.