The invention discloses a concrete medium erosion resistant additive based on an in-situ reaction encapsulation technology and a preparation method. Core components of the concrete medium erosion resistant additive based on the in-situ reaction encapsulation technology are encapsulated active components, the encapsulation is to absorb and encapsulate active components in an interlayer structure byvirtue of an encapsulation component, and the ratio of the encapsulation component to the active component is 0.5 to 8. According to the invention, inorganic nano-silica particles with high hydrationreaction activity are encapsulated on the basis of a coating technology, and then the ionization, release and reaction of the inorganic nano-silica particles in a concrete hydration microenvironmentare packaged at a later stage, so that the long-term chloride ion diffusion coefficient of the concrete is obviously reduced by virtue of optimization of the concrete microstructure in the service process, and the concrete medium erosion resistant additive based on the in-situ reaction encapsulation technology and the preparation method are provided for a concrete durability improvement technology.