The transmitted intensity and the modulation depth of plane polarized light are implemented to investigate influences of the low frequency external magnetic field on the ferrofluid film. The frequency of AC magnetic field is modulated from 5Hz to 400Hz, and the thickness of ferrofluid films are changed as 12 μm, 25 μm, 140 μm and 520 μm, and the mass concentration of magnetic nanoparticles is 5 %, 10 %, 20 % and 33.6 %, respectively. The experimental results show that (1) the mean value of the optical transmittance of ferrofluid film is decided by the thickness of ferrofluid film and the mass concentration of nanoparticles, but the frequency of magnetic field has no direct relation with it; (2) under a given magnetic field intensity, the optical modulation depth of ferrofluid film is determined by the film thickness, the nanoparticle concentration and the magnetic field frequency. It is further found that the single relaxation characteristics of magnetic nanoparticle, whose size is smaller than the critical value of the single magnetic domain, is the important factor of the frequency properties of nanoparticle in ferrofluid. These results may be beneficial to applications of ferrofluid in magneto-optic sensors.