The invention discloses a 90-degree optical mixer based on a hybrid plasmon waveguide structure. The 90-degree optical mixer is composed of a single 4*4 multimode interference coupler (MMI). The hybrid plasmon waveguide structure consists of a dielectric waveguide layer, a slit dielectric interlayer on the dielectric waveguide layer, and a metal layer on the slit dielectric interlayer. When lightenters the hybrid plasmon waveguide, the special structure of the hybrid plasmon waveguide can excite a hybrid plasmon mode and confines the mode in the slit dielectric interlayer so that the propagation distance under the same optical path can be greatly reduced and thus the size of the device is greatly reduced. Thus, compared with a conventional dielectric waveguide mixer, the 90-degree opticalmixer is reduced in size from millimeters to micrometers, provides a new structural solution for highly integrated photonic devices and has far-reaching effects on the further integration of photonicdevices.