The subject of the research is the diagnosis of the technical condition of the plate of knife refiners. The technical diagnosis of a knife plate consists of determining the stability of the inter-knife gap and the degree of the plate wear. The paper explores the dynamic effects of a plate on a fibrous material. It is shown that the main source of high-frequency vibration of the stator is pressure pulses that occur when the rotor and stator knives cross. The influence of the rotor speed and the number and angle of intersection of the plate knives on the nature of the stator vibration spectrum is studied. It is concluded that the intensity of the effect of the plate on the fibrous material increases from the centre to the periphery. A formula is proposed for determining plate frequencies taking into account the Doppler effect. The vibration diagnostics of the rotor beats is based on the resonance-demodulation method. Estimating the depth of the amplitude modulation of vibrational acceleration of the stator at the maximum plate frequency with harmonics of the working frequency, it is possible to judge the amount of beating of the rotor plate. A device was developed for vibration diagnostics of the beats of a rotor plate of a disk mill. The developed methods and tools can be used in other industries, for example, in mining and metallurgy. © Published under licence by IOP Publishing Ltd.