Fiber measurement technology has many advantages, such as no electromagnetic interference, good insulation and small volume. A kind of fiber current sensor is giver, which bases material mechanics and electromagnetic induction, and also makes a cantilever beam and F-P together for measurement. Phase demodulation way is adopted in this paper. The sensor principle and measurement principle of F-P are analyzed. This paper builds up the mathematical model of the F-P gap and gave a current measurement equation. At last some experiments are done to validate it. Advantages of this sensor are small volume, high sensitivity and comoact structure.