In ferromagnetic resonance experiment, under the action of a stable magnetic field, the change of the permeability of samples to be measured will cause a shift of the resonant frequency ω of the resonant cavity. that is the dispersion effect, which has a significant impact on the ferromagnetic resonance curve. Transmission resonator is applied to measure the ferromagnetic resonance curve in experiment, and obvious dispersion effect appeared. Based on the influence of the ferromagnet on the natural frequency, the perturbation theory is adopted to explain the cause of the dispersion effect and its influence on the ferromagnetic resonance curve. Thus it solves a difficult point in the teaching process of ferromagnetic resonance experiment.