Based on Faraday effect, miniature and agile magnetic field sensor were demonstrated by using a dual-polarization fiber grating laser. To develop suitable package to optimize the sensitivity for magnetic field, it is necessary to investigate the spatial response of a dual-polarization fiber grating laser to magnetic field. Experiment measurements for the longitudinal spatial response to magnetic field were performed on a fiber grating laser with a length of 21 mm in magnetic fields of 0.5 T, 0.75 T and 1 T and field widths of 3 mm, 5 mm and 7 mm. It shows that the spatial response is not uniform along the longitudinal direction of the dual-polarization fiber grating laser. The spatial response peaks appear at the center of the laser cavity and show as a symmetric Gaussian distribution along the longitudinal direction of the laser. Such distribution characteristics of the spatial response are related to the distributed Bragg reflecting.