The invention provides a method for preparing cement-based water-based composite nano-silicate inorganic functional sol, which is mainly characterized by comprising the following steps of: heating and stirring alkali silicate serving as a raw material, and slowly adding methyl sodium silicate dropwise; stirring for certain time, cooling to normal temperature, adding an organic amine substance, and stirring to ensure that the organic amine substance is subjected to full crosslinking reaction; adding nano-particles, such as SiO2 or CaCO3 and the like, which are subjected to surface modification, and fully stirring to ensure that the nano-particles which serve as a dispersion phase are completely dispersed in the sol; adding various functional additives, and stirring to ensure that the added functional additives are fully dispersed in the sol; and slowly adding aqueous solution of magnesate dropwise, heating and stirring to ensure that magnesium ions are effectively crosslinked in the sol. A coating is coated on the surface of concrete, has high film forming performance, forms a compact waterproof layer on the surface, permeates through the surface layer of the concrete to fill the capillary channels of the concrete, and improves the hydration and the internal crystallization of the concrete. Therefore, the hardness, the abrasion resistance and the like of the concrete are improved. The method has the advantages of simple preparation process, environmental protection, no pollution, contribution to enlargement and large scale production.