Abstract: The guide wheel shaft of a linear motor pumping unit in an oil field fractured. The failure reasons were analyzed by macroscopic inspection, non-destructive test, chemical composition analysis, metallographic inspection, scanning electron microscope, tensile test and impact test. The results show that there were brittle Widmanite and martensite structures in the fractured wheel shaft, and the crack was easy to initiate at the welding joint with the web effected by the welding thermal stress. Under the alternating load caused by the reciprocating movement of the pumping unit, the cracks continued to propagate, and eventually led to the fatigue fracture of the wheel shaft. It was recommended to strictly control the heating temperature and cooling speed during heat treatment of the wheel shaft, optimize the welding process, and strengthen the quality supervision and delivery inspection of the wheel shaft.