On the basis of the controllable refractive index of a magnetic fluid,a novel fiber optic F-P electromagnetic sensor characterized by simple structures and no moveable parts is developed by using the magnetic fluid as the medium in a F-P interference cavity. Then,a signal demodulation method based on fiber Bragg grating wavelength scanning is proposed. The principle of electromagnetic-controlled refractive index of the magnetic fluid is described and the structure of the sensor system is given. Preliminary experiments are carried out,obtained results show that the thickness of magnetic fluid film and environmental temperature all affect the measurement results. When the thickness of magnetic fluid film is 12.7 μm, there is a fairly good linearity of the measurement characteristic.