Abstract: The optical relaxation properties of magnetic fluids are investigated in details. In the experiments, the light beam goes through the magnetic fluid samples perpendicularly. Experimental results indicate that the falling relaxation time is shorter for large volume fraction sample, while the rising relaxing time is longer for large volume fraction sample. For a certain sample, the rising relaxation and falling relaxation time increase with the magnetic field and the number of turning magnetic field on and off. Besides, the rising relaxation and falling relaxation time tend to saturate when the number of turning magnetic field on and off is very large. The corresponding physical mechanisms are clarified. These results are convenient for researchers to study optical properties of magnetic fluids deeply. The results provide a guideline for designing the potential photonic devices and are helpful for improving their quality.