Abstract: During the processing of the AA3003 aluminum alloy radiator connecting piece, there were large areas of black spots appeared on the surface. Through macro observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, the reasons for black spots on the surface of connecting piece were analyzed. The results showed that when cleaning and lubricating the equipment, the synthetic engine oil accidentally drop on the surface of connecting piece. During the brazing process, the synthetic engine oil undervent coking reaction to form black spots.