Abstract: Biotite is a common rock - forming mineral and widely exists in reservoir，it is stable in solution rich in K
+ and easy to hydrate in solution poor in K+，biotite hydration may transform into hydrobiotite，vermiculite，montmorillonite，chlorite，kaolinite and so on.Based on chemical structure and crystal form of biotite，the authors of this paper analyze mechanism of biotite hydration，biotite hydration in essence is a procedure of releasing K+ interlayer.Hydration falls into edge stage with the characters of cleavage dehiscence and complete stage with features of cleavage disappearance.Once biotite hydrate occurs in reservoir，crystal structure will swell，particles on the edge will gradually drop off，migrate with hydrodynamic dispersion，block the throat and damage reservoir.Thus，water injection in reservoir with biotite minerals is suggested to boost concentration of K+，avoid water and flow velocity sensitivity due to generation of biotite hydration and protect reservoir.

Key words: biotite, hydration, mechanism, water injection, damage