Abstract: A certain 27SiMn steel displacement jack cylinder had defects such as holes, falling blocks, and cracks at the bottom. The reasons for the cracking of the bottom of the jack cylinder were analyzed using methods such as macroscopic observation, low magnification acid corrosion inspection, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that during rough machining, due to dimensional errors, repair welding was carried out. Improper operation during repair welding resulted in incomplete fusion of the material, ultimately leading to cracking of the bottom surface of the cylinder bottom inner hole.