Abstract: A spring pull rod of constant spring support with three connecting rods of a thermal power unit was broken. The fracture cause of spring pull rod was analyzed by means of macro and micro morphology observation, chemical composition analysis, microstructure observation, mechanical property testing and force simulation. The results show that the material of fractured spring pull rod was non-quenched and tempered 45 steel with poor fatigue performance. Stress concentration at the root of the first thread was caused by the insufficient thread processing length and surface processing defects of the spring pull rod, which formed the crack source. During the operation of constant spring support with three connecting rods, the internal spring pull rod was deflected and subjected to additional bending moment. Under the joint action of axial tension, bending load, and additional alternating stress produced by pipe vibration, low stress high cycle fatigue fracture of the spring pull rod occurred.