The characteristics of grating structure in magnetic field measurements based on differential group delay of fiber Bragg gratings (FBG) are analyzed. Theoretical simulations are realized using the coupled-mode theory and transfer matrix method. The effects of grating parameters (physical length,index modulation) of uniform FBG on measurement range and sensitivity are analyzed. The impacts of chirped,phase-shifted and apodized gratings on the wavelength dependency of differential group delay are also monitored. The experimental measurements are conducted on FBG inscribed by means of the phase mask technique. The experiments demonstrate that the phase-shifted gratings can obviously improve the sensitivity of sensor system which match well with the simulations.