Abstract: A batch of planetary gears used as raceways were subjected to grinding burn detection, the black spots were found on the inner hole surface. Macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination, black spot number statistics and other methods were used to analyze the cause of black spots on the inner hole surface of the planetary gear. The results show that there were non-metallic inclusions on the inner hole surface of the planetary gear. During the grinding burn detection process, the non-metallic inclusions were corroded and formed black holes, the macroscopic performance were black spots.