Abstract: Cracks were found in a batch of special products of splicing sleeve bodies when they were magnetic particle inspected after black phosphating, and means such as macro and micro fracture analysis, X-ray energy spectrum analysis, metallographic examination and hardness test were used to analyze the cracking reasons. The results show that the material of the splicing sleeve body was high hardness steel with high hydrogen embrittlement sensitivity. The assembly sequence of the parts violated the normal process, with first assembling by hydraulic machine, and then correcting the degree of symmetry between the dovetail groove and the hole axis. The abnormal assembling procedure increased the residual stress of the inner hole wall sheet R transition position of the parts. At the same time, the internal stress of the part itself was rather high, so hydrogen infiltration cracking happened and hydrogen embrittlement cracks formed during the process of pickling and phosphating.