The utility model discloses a sub-wavelength plasmon microcavity optical coupling structure, which is used for modulating propagation direction and optical field distribution of incident light through a plasmon microcavity, so that the incident light is limited to propagate in the microcavity, the escape of the light is reduced, and the utilization rate of photons is improved. The incident light field is concentrated in the microcavity so that the intensity is greatly enhanced, and a photoelectric detector with high response rate can be formed by clamping photoelectric conversion materials in the microcavity. The coupling structure is composed of a metal grating layer, a photoelectric conversion activation layer and a lower metal reflection layer, wherein the metal grating layer is formed by upper-layer periodic metal strips. The sub-wavelength plasmon microcavity optical coupling structure has the advantages that photons entering the microcavity propagate transversely and form standing waves by utilizing the mode selection effect of the electromagnetic wave near-field coupling microcavity formed by resonance of plasmons between the upper metal grating layer and the lower metal reflection layer, the optical field energy is concentrated, the equivalent light absorption length is increased, and the response rate of the detector is greatly improved.