The detection of frequency is one of the most important aspects in the fault diagnosis of mechanical equipments. The amplitude of mechanical vibration is small when the mechanical equipment is in normal operation. According to the law of conservation of energy, a kind of special vibration measuring sensor is designed which is a variable distance capacitive sensor and it can achieve mechanical vibration amplitude amplification. In the hardware processing circuit, a fully differential amplifier circuit is used to make secondary amplification of electrical signal, and the amplified analog signal is sampled by high resolution ADC. Nanosecond precision time is achieved through software subdivision algorithm, so as to realize the high precision of frequency measurement.