The measurement of magnetic field through the polarization dependent loss (PDL) of phase-shifted grating is a more novel approach. It can be used to measure the pulse alternating strong magnetic field. The key issue of fiber grating sensor is the cross-sensitive. Ignoring the inherent birefringence of gratings,the principle of magnetic field sensing is derived and simulated by transfer matrix method. The influence of the temperature,axial stress and pressure on the transmission window of phase-shifted grating is theoretical analyzed,and the corresponding changes of the PDL peak value are simulated. The results show that the wavelength of transmission window peak value changes linearly with temperature and axial stress,PDL spectrum horizontal lines shift,and the result of magnetic field measurement will not be affected. The refractive index changed with pressure causes the irregular change of PDL spectrum,having a serious impact on the magnetic field measurement.