Tailing samples were collected by drilling in No. 1 tailing dam from the Dexing mine. The multi-element analytical scheme were applied. The tailing size was determined by the laser grain size analytical instrument, and the mineral form was determined by X-ray powder diffractometry. Some conclusions have been reached: The major particles of the tailing are silty, with quartz and sericite as the main minerals, while the content of metal minerals is less than 1%. The water-sorting action and supergene geochemical process in the tailing dam combined with changes in ore grade and mineral processing caused uneven distribution of elements and particle sizes of tailing. The tailing particle size decreases in the direction of the tailing discharge, and the elements can be divided into 3 zones: Cu, Fe, Mo→SiO₂, Au→Al₂O₃, Tl. During the supergene geochemical process, the sulfide was weathered into oxides,which led to the significant decrease of the pyrite content, with the loss of Cu, Mo, S, Fe, Ca and C in tailing. Meanwhile sericite changed into illite.