Abstract: In vibration test, the balanced bellows of an aviation pipeline compensator cracked. The causes of cracking of bellows were analyzed by macroscopic observation, fracture analysis, chemical composition analysis, metallographic examination and hardness test. The results show that the cracking property of the balanced bellows was fatigue cracking; the welded joint between the inner bellows near the crack end on the balanced bellows and the connecting ring was not fused, so that the internal gas entered the interlayer of the double bellows. There were surface defects in the longitudinal weld of the outer surface of the first peak of the balanced bellows, which would produce stress concentration. The crack originated from the stress concentration of the inner surface of the bellows. Under the combined action of internal gas pressure and cyclic stress of vibration test, the crack continued to expand to the outer surface and along the circumferential direction, and finally fatigue cracking occurd.