The invention relates to a frequency detector for measuring the conductivity of liquid through adopting a Venn bridge oscillating circuit. The frequency detector consists of an MEMS device and a circuit together; the MEMS device comprises two equivalent capacitors manufactured by silicon microprocessing technology; a medium of the capacitor is detected liquid; and the MEMS device is fixed on a PCB board and is connected with a circuit to form the Venn bridge oscillating circuit and a posttreatment circuit for signal acquisition. In detection, the detected liquid is injected into the frequency detector or the frequency detector is put into the detected liquid; for corresponding to liquid with different conductivity, the capacitance value of the frequency detector is different; and the oscillating frequency of the Venn bridge oscillating circuit is different; therefore, through detecting the oscillating frequency of the Venn bridge oscillating circuit, the conductivity of the liquid is measured. The detector has the advantages of high sensitivity, strong antijamming capacity, convenient detection and the like, utilizes a minitype device manufactured through utilizing the MEMS technology, is easy for micromation of a detection instrument and is also convenient to be integrated with other sensors to form a multi-parameter minitype detection instrument.