The mechanism is put forward that the axial magnetic field parallel to the wave-guide of the LiNbO3 modulator may affect the half-wave voltage. The electric field acting on the electrode of the LiNbO3 crystal may bring linear birefringence to the polarized light propagating in LiNbO3 modulator, and the axial magnetic field may bring circular birefringence to it, too. So the acting result of the two types of birefringence makes the half-wave voltage vary with the magnetic field. The maths model about the variance that the half-wave voltage varies with the magnetic induction outside is set up. The simulation result and the examination result show that as to the light source whose wavelength is 1.30μm, when the magnetic induction changes about 20 Tesla, the half-wave voltage of LiNbO3 modulator may decrease 2.13%.