The invention provides a capacitive sensor based on resonant frequency measurement and a working method and an application thereof. The capacitive sensor includes a sensor probe forming a flat
capacitor together with a detected pole plate, an equal amplitude frequency stabilizing sinusoidal oscillator connected with a probe end of the flat
capacitor, an equal amplitude frequency stabilized sinusoidal oscillator, a
local oscillator used for outputting amplitude and frequency-stable
local oscillator signals, an orthogonal demodulator used for demodulating an oscillating
signal into an in-phase component and an orthogonal component through the
local oscillator signal, and an orthogonal phase measuring device used for calculating an inverse
tangent function of the orthogonal component and thein-phase component to obtain the phase and the frequency of a
frequency difference signal, wherein a detected pole plate end of the flat
capacitor is grounded, the equal amplitude frequency stabilizedsinusoidal oscillator is used for outputting an oscillating
carrier signal with the stable amplitude and the
frequency offset corresponding to the displacement amount or the
area change amount of thedetected pole plate, and the displacement amount or the
area change amount of the detected pole plate is obtained based on the relationship between the frequency of the
frequency difference signal and the
capacitance change amount. The capacitive sensor is advantaged in that precise measurement can be achieved in a quite wide range of sampling frequencies.