A new magnetic field measurement based on the Faraday effect and the measurement of differential group delay (DGD) in fiber grating was proposed.When the magnetic field was applied, the propagation constants of the two circularly polarized light in fiber grating were changed by the Faraday effect, then insulting in the change of the DGD of fiber grating.The simulations showed the linear relationship between the peak value of DGD and magnetic field in certain range. The influences of fiber grating on the measurement performance were analyzed. The measurement sensitivity was 1.3 ps/T and the smallest magnetic induction is 10-5 T using the optical vector analyzer with precision of 10-5 ps. The experimental results agree well with the theoretical analysis.