The invention belongs to the technical field of alumina sol, and discloses alumina sol with higher activity and higher stability and a preparation method thereof. The preparation method comprises thefollowing steps that (1) a red phenolphthalein reagent solution is added into an aluminum solution and stirred for 5-180 minutes at the temperature of 20-80 DEG C to form floccule, wherein the molar ratio of red phenolphthalein to Al is (0.001:1) to (0.1:1); (2) the floccule is added into water, and subjected to washing and separation, and an intermediate product A is obtained; and (3) the intermediate product A is dispersed in an orange water solution containing sodium sulfonate salt and stirred for 10-180 minutes at the temperature of 10-60 DEG C, and the alumina sol is obtained, wherein themolar ratio of orange sodium sulfonate salt to Al is (0.001:1) to (1:1). The invention further provides the alumina sol obtained by adopting the preparation method, the activity is high, the stability is good, the pH value is lower than 5.5, the viscosity is less than or equal to 600mPa.S, the alumina sol is particularly suitable for aluminum cladding of ternary anode materials of lithium batteries, and improvement of the performance of lithium ion batteries is improved.