Abstract: A New magnetic field measurement with fiber grating was analyzed in theory and experiment. When the magnetic field applied, the transmitted constants of two circularly polarized light in fiber grating will changed by the Farady effect. And so does the chromatic dispersion（CD） and differential group delay（DGD） of fiber grating. The simulation show that there are linear relationship between the magnetic circular birefraction and the ratio of the CD and DGD if the magnetic field is not strong. The value of magnetic circular birefraction can be derived from the measured value of CD and DGD using character, and the value of magnetic field can be derived. The measurement sensitivity is 6. 58522e^-18 m/Gs and the precision of this method is 2Gs using the optical vector analyzer with 10^-17 precision in experiment and those agree well with the theoretical analysis.