A frequency based sensor is presented to measure the conductivity of fresh water. A MEMS based conductivity sensor is designed to form the RC-Wien-Bridge Oscillator, and the conductivity of tested sample could be obtained according to frequency of the Oscillator by calibrating the frequency to the conductivity of standard liquid. In this paper, system description and electronic simulation are presented with an emphasis on the simulation of actual capacitance and frequency-capacitance characteristics of fresh water conductivity sensor. The results indicate the frequency range dedicated for fresh water conductivity measurement is from 90 kHz to 434 KHz. It is promising to measure the conductivity of fresh water with this kind of frequency based sensor.