A comprehensive investigation was conducted on the leakage at the root weld of impulse line of the radiant coil in cracking furnace of a 1.2 Mt/a ethylene plant. Through macroscopic morphology, material testing, metallographic examination, electron microscopy scanning and energy spectrum analysis, it was found that the cracking at this location was due to the decrease in the toughness of small tube structure, resulting in fatigue cracks under structural stress. The reason was that severe stress concentration occurred at the connection between the main pipe and the branch pipe. Material sensitization occurred during long-term service at high temperature, leading to microcracks at the weld seam. These cracks gradually expanded from the outside to the inside, ultimately forming through-thickness cracks and causing leakage.