Abstract: In this paper, a signal demodulation method by software for high sensitivity fiber optic sensor of magnetic field is introduced. The measure pricision of the sensor is 1 nT, the minimun detectable field per unit bandwidth is 2.86×10-10 T/Hz1/2 and the dynamic range is 60 dB for the sensor. This work is very useful for detecting weak magnetic field technology and the research of interferential fiber optic sensors.