The intensity of light transmitted through a thin ferrofluid film could be modulated by an applied magnetic field. After the magnet was switched on, the light transmission coefficient decreased first to a minimum value, then raised up to a stable level, forming a valley in the time section. The stronger the applied magnetic field is, or the higher the particle volume fraction is, the lower the valley value of light transmission coefficient. After the magnetic field was switched off, the final stable value of light transmission coefficient was affected by the remanence of magnet. The relaxation process of light transmission through the ferrofluid film could be repeated by continuously switching on and off the magnet, but the highest transmission values and the lowest transmission values can be stable without drift only after many cycles. The behavior of light transmission coefficient should be related with the particle chains’ forming, lengthening, moving, and breaking in the ferrofluid film.