Abstract: During the reciprocating durability test of a batch of the office chairs, the chair backs fractured to failure. By means of fracture analysis, chemical composition analysis, hardness testing, inclusion inspection, metallographic inspection, scanning electron microscope (SEM) and energy spectrum (EDS) analysis, the reasons why internal springs of office chair backs fractured in reciprocating durability test were analyzed. The results show that:the hardness distribution of the spring surface was uneven, and there were surface decarburization on surface and the inclusions in subsurface; these defects destroyed the continuity of the matrix, leading to the crack source during reciprocating durability test; the cracks propagated rapidly under the multiple stress condition, and finally fatigue fracture occurred to the springs.