Abstract: A certain type of copper-aluminum transition clamp had five consecutive cracking accidents. The cracking reason of the clamp was studied by means of macroscopic observation, energy spectrum analysis and non-destructive testing. The results show that due to improper welding process control, the clamp had incomplete penetration defects. The thermal expansion coefficients of copper and aluminum were different. Under the influence of ambient temperature and load changes, the defects expanded and eventually led to cracking of the clamp.