Abstract: The mechanical seal pair of the primary pump of a circuit in a nuclear power station suddenly failed during normal operation. The macro-morphology, composition, metallography, SEM micro-fracture, thickness, micro-hardness and operation history of the failed seal rings were analyzed. The results show that there is no obvious manufacturing defect in the seal rings. The failure reason is that the compression of the moving ring spring is too large and the clearance is too small. As a result, the dry friction of the moving ring caused the initial thermal crack. In the later operation, the thermal cracks extended to cross each other under alternating load, and the falling of debris at the intersection led to abrasive wear of the static ring, finally causing the sealing clearance to exceed the threshold.