Abstract: The transverse fracture reason of a Q345B steel fishtail forging was analyzed by macroscopic observation, chemical composition analysis, scanning electron microscope analysis, mechanical property test and metallographic examination. The results show that there were many and large non-metallic inclusions in Q345B steel, resulting in a significantly reduced impact energy of fishtail forging, which seriously affected the toughness of the material. When the fishtail was impacted by external force, brittle fracture occured.