Abstract: Introduces working principle and structure of a indication and alarm device for over water content rate in piped crude oil. The electric amount in a capacitance in sensitive probe composed of emitting antenna and the corresponding oil transport pipe case will vary with water content rate in crude oil and the probe is also used as a modulate capacitance of a resonance circuit. The resonance circuit is coupled by inductance and obtains oscillating voltage from a stable crystal oscillator. When the capacitance of the sensitive probe is changed, the impedance of the oscillator circuit will be changed consequently, e.g. changes in oscillating voltage will occur. The change will be transferred into D.C. voltage changeable output through detecting circuit and displayed.