The paper using the characteristics of reflected electromagnetic waves proposes a principle of measuring the porosity of a fluidized bed in a boiler. Features of the reflecting properties of a fluidized bed representing a multitude of solid particles are examined. The use of the amplitude of a signal reflected from the particles is justified with account of their geometric dimensions and electrophysical properties. The nature of the relationship between the intensity of the reflected electric field and the multitude of solid particles is studied with due regard to the ratio of a wavelength to a diameter of dielectric particles. An analysis of the desired signal reflected from the fluidized bed is performed on the basis of amplitude modulation. A recommendation is given on transformation and statistical processing of information on the porosity of an amplitude-modulated signal.