The invention relates to preparation of positive material for lithium ion batteries, and in particular relates to a preparation method of lithium cobaltate positive material coated with nanometer aluminum oxide solid phase. The method comprises the following steps: 1, preparing lithium cobaltate, namely, mixing tricobalt tetroxide with lithium carbonate, feeding 1400-1500ppm of basic magnesium carbonate, sintering the mixture at the temperature of 950-980 DEG C, naturally cooling in a furnace, and crushing to obtain lithium cobaltate; and 2, enabling lithium cobaltate to be coated with the nanometer aluminum oxide solid phase, namely, firstly feeding lithium cobaltate and nanometer aluminum oxide powder having the particle size D50 of 20-22nm into a ball mill, feeding grinding balls with the mass twice that of the material, sintering the material treated by ball-milling at the temperature of 780-800 DEG C, then naturally cooling in the furnace, and crushing the cooled material in a crusher to obtain the lithium cobaltate positive material coated with the nanometer aluminum oxide solid phase. After the preparation method is adopted, the cycle performance and the high-temperature performance of the lithium cobaltate positive material can be effectively improved, the coating uniformity of the lithium cobaltate positive material is good, and the product performance is stable.