Through analyzing the characteristics of polarization extinction ratio(PER) in magneto-optic fiber Bragg gratings(MFBGs),a new magnetic field measurement method based on peak value of PER is proposed.The OptiSystem simulation shows that the peak value of PER is linear with the logit value of applied magnetic field when the grating coupling coefficient is much larger than the magneto-optic coupling factor,which is in accordance with the theoretical analysis.In addition,the analytic expression of peak value of PER associated with the length of grating,the modulation depth of refractive index and the Verdet constant is also given,with convenient application to the performance analysis of magnetic field measurement.