Abstract: By means of chemical composition analysis,hardness test, microstructure examination and SEM/EDS analysis,the cracking reason of the tube used in anti-shock instrument of automobile during assembling process was analyzed.The result show that the tube cracking was plastic cracking, and too big assembly stress was the major cause for the cracking.Local roase microstructure, asymmetric grain size and low hardness were the other causes which resulted in the cracking when the tube was assembled.