Abstract: Mucin5AC in ocular surface is a macromolecule gel\|forming secretory mucin expressed by conjunctival goblet cells, and dissolved in aqueous tear film layer. MUC5AC has many functions such as clearing  fragments that exist in ocular surface, keeping ocular surface from harm, lubricating, promoting the tear drainage, keeping the stability of the tear film, preventing the invasive pathogens and stopping bleeding, et al. MUC5AC related with many ocular diseases. The expression of MUC5AC can be affected by many conditions, such as infectious and immunologic diseases, LASIK, corneal contact lens wearing, and vitamin deficiency, which may  lead to clinical symptoms, or reduce the ability of resisting the invade and attack from outside.Thus, diseases like dry eye, allergic disease, and degenerative disease may occure. (Int Rev Ophthalmol, 2014, 38: 160-164)