The invention relates to a coated lithium cobalt oxide positive material with high capacity, low residual alkali and low pH value, and a preparation method thereof. The method is mainly characterized in that cobaltous hydroxide is coated on the surface of nickelous hydroxide by utilizing a coprecipitation method, so as to form a precursor of a gradient material [Co(OH)2]x.[Ni(OH)2][1-x], wherein x is smaller than or equal to 0.35 and greater than 0; the precursor is mixed with a lithium source, then high-temperature sintering is carried out at oxygen atmosphere so as to obtain the lithium cobalt oxide positive material [LiCoO2]x.[LiNiO2][1-x] with a gradient structure. The prepared lithium cobalt oxide positive material with the gradient structure disclosed by the invention is low in pH value, excellent in cycle performance, simple in product process, easy to control, low in cost and suitable for volume production.