The polarized light transmittance of magnetic fluids under longitudinal magnetic field and the influence of magnetic fluids concentration, dielectric constants of the remanent liquid phase within the magnetic fluids, the ratio of dipole moment to the thermal energy of single magnetic nanoparticle and the number of magnetic nanoparticles per agglomeration are investigated numerically according to the analytical expression when considering both the geometric shadowing effect and the Faraday rotation effect. Theoretical results indicate that the magnetic fluids concentration, dielectric constants of the remanent liquid phase within the magnetic fluids and the ratio of dipole moment to the thermal energy of the single magnetic nanoparticle affect the polarized light transmittance apparently. The polarized light transmittance of magnetic fluids increases linearly or oscillates with the strength of the longitudinal magnetic field for low or high concentration samples, respectively. In certain ranges, the polarized light transmittance of magnetic fluids increases with dielectric constants of the remanent liquid phase and the ratio of dipole moment to the thermal energy of single magnetic nanoparticle. While the polarized light transmittance is almost independent of the number of magnetic nanoparticles per agglomeration. The distinct difference of the parameterdependent polarized light transmittance at low and high magnetic field regions is obtained. The applications to several photonic devices based on the polarized light transmittance of magnetic fluids under longitudinal magnetic field are proposed.