Using cadmium acetate and sodium sulfuret as Cd and S sources, CdS samples were synthesized by chemical precipitation method. The conditions of supersaturation and aging time had been investigated, which could influence the activity of CdS for photocatalytic degrading methyl orange. The products were characterized by X-ray powder diffraction and field emission scanning electron microscopy. The CdS samples synthesized under low supersaturation showed better photocatalytic activities. A phase transformation of hexagonal CdS to cubic CdS was observed when CdS precipitation aging and the presence of proper amount of hexagonal CdS should improve the photocatalytic performance of cubic CdS.