Abstract: The crankshaft of a certain air compressor was broken. The causes of crankshaft fracture were analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, mechanical property testing, metallographic examination. The results show that the hardness at crankshaft R was low, the size was small, and there were obvious machining tool marks, resulting in low fatigue strength at crankshaft R and stress concentration, and the initiation of fatigue cracks. Under the action of alternating loads, the crankshaft experienced fatigue fracture.