Abstract: Black contamination was found on the strip steel surface after using a type of formation fluid in a steel mill’s cold rolling continue annealing line. Aiming at this problem, the solvent dissolution tests, scanning electron microscope and energy diffraction spectrometer were used to analyze the generation causes of the black contamination. The results show that the black contamination was caused by the annealing slag. Finally the countermeasures were proposed according to the causes.