Incomplete phenomenon was found on the sealing surface of the powder metallurgy exhaust valve seat inserts in the factory inspection before delivery. Through chemical composition analysis, hardness testing, density testing and metallographic analysis, it was found that the chemical compositions, hardness, density and microstructure of the failure seat inserts were all very close to those of the normal seat inserts and they were all in the range of control requirements. By observation with Nano CT imaging scanning system and scanning electron microscopy (SEM), no obvious internal defects and pores were found, but bump marks were found near the surface defect position, and effective sintered junction did not form at this position. Through the inspection of the production process, it was found that the sponge cushion on the transmission rail damaged, so the green compact of the seat inserts bumped with the transmission rail in the discharge process after pressing, which resulted in the damage. So incomplete phenomenon appeared on the sealing surface of the seat inserts after sintering.