Pickling is a key step in the manufacturing process of cold-rolled products， and the surface state of metal materials and the factors such as concentration， temperature and treatment time of acid liquor have a significant impact on the pickling effect. Therefore， the composition and structural characteristics of oxide scale at different parts in width direction of SDX51D strip steel before and after tension leveling and sacle breaking treatment were studied by X-ray diffraction， scanning electron microscopy and energy spectrum analysis. And the influences of different parts in width direction of strip steel， before and after tension leveling and sacle breaking， pickling concentration and pickling temperature on the end time of pickling were investigated by electrochemical methods. Furthermore， the end time of pickling was determined by potential derivative first zero method. The research results indicate that the compositions of oxide scale at the edges along width direction of SDX51D strip steel are Fe2O3 and Fe3O4， and the compositions in the middle are Fe3O4 and FeO. The thickness of oxide scale in the middle is 3 μm thinner than that of the edges， and the thickness distribution is uneven. It is not tightly bonded to the matrix， and the pickling time is 10 s shorter than that of the edge. The cracks of oxide scale increase after tension leveling and sacle breaking， and the thickness is reduced by an average of 2-3 μm， which is conductive to pickling. Compared with the time before tension leveling and scale breaking， the pickling time is reduced by 20 s. Thus， considering the pickling effect of SDX51D strip steel comprehensively， the recommended pickling temperature is 80 ℃， and the pickling concentration is 10% during the pickling process.