The invention discloses an optical coupling unit of a quantum well long-wave infrared detector grating waveguide micro-cavity, structurally characterized by adopting an interlayer structure of a metal back reflection layer, a dielectric layer and a metal grating. The optical coupling structure has the advantages that (1) the coupling efficiency of incoming light is improved by adopting a metal array structure; (2) the electric field intensity of a quantum well is greatly improved by limiting an optical field to propagate in a mini space between the metal grating layer and the metal back reflection layer; (3) a vertical electric field component has favorable evenness in a longitudinal direction by adopting a similar microstrip antenna structure; (4) the metal structure is not only an optical coupling unit, but also is used an upper electrode or a lower electrode; and (5) the optical coupling unit is easy to manufacture and suitable for manufacturing photosensor elements with large area arrays.