The invention belongs to the technical field of the production process of activated kaolin and in particular relates to a method for preparing activated kaolin by washing kaolin with water. The method comprises the steps of preparing a kaolin filter cake, preparing kaolin dry powder, thermally activating kaolin, and preparing activated kaolin by performing depolymerization and modification on the thermally activated kaolin. Deep treatment is performed on kaolin resources by use of processes such as appropriate-temperature calcination, efficient depolymerization and dispersion and modification on the basis of the traditional treatment process of washing the kaolin with water, therefore, the activated kaolin having the characteristics of volcanic ash can be prepared; the activated kaolin can be applied to the fields such as a high-grade concrete additive, a cable filler, a water-based coating and the like, and has the characteristics of short setting time, high early strength, shrinkage compensation, no corrosion to reinforcements, high resistivity and the like; as a result, the valued added of the kaolin resources can be increased, and the activated kaolin has enormous market space potential.