The invention discloses a waveguide-based adjustable microwave isolator loaded with ferrite, and relates to the technical field of microwave materials. The waveguide isolator with an adjustable working frequency band is designed by introduction of a one-way electromagnetic boundary state into the waveguide; periodically-arranged ferrite columns are coupled with electromagnetic waves under the function of an externally-applied bias magnetic field to excite a magnetic surface plasmon so as to form the one-way electromagnetic boundary state on the surfaces of each group of ferrite columns; static magnetic fields with opposite bias directions are externally applied to the two groups of ferrite columns in the axial direction respectively; the waveguide isolator provides different responses to the electromagnetic waves in different incident directions which are shown as conduction and blocking characteristics respectively; by changing the magnitude of the bias magnetic field, the resonance frequency of the magnetic surface plasmon can drift, so that the working frequency band of the isolator can be adjusted without changing the appearance, thereby realizing the adjustability of the frequency; meanwhile, heat dissipation of the ferrite columns is rapid; and the microwave isolator is simple in structure.