Through a great deal of experiments , a failure axle of air -breathing pump on the top of decompression tower was analyzed and researched in details , and then its causes of inactivation was found out.Chemical components, mechanical properties, metallurgical structure and corrosion substance in the pit of the pump axle were tested.By analyzing the results of tests, three main causes of the axle inactivation are concluded.Firstly , non -metallic inclusions in the microscopic structure go heavily beyond the standard value .Secondly , microscopic structure doesn' t conform to technical requirements.Finally , in the bottom of pump axle' s key way , sectional dimension changes greatly , the key way' s root is sharp-angled, and there exist many corrosion pits.These defects act as sharp gaps .Under the effect of alternately rotating and winding load , notch effects happen heavily along these defects, and a very high local stress concentration is formed, which leads to failure due to pump axle' s fatigue fracture .On the basis of confirming failure causes, a proposal is put forward that the relevant measures are taken to prevent pump axle from fatigue cracking too early and prolong its operating life .