The ferrofluid film(several micrometers thick), located between two glass plates, was placed vertically in the uniform magnetic field produced by the Helmholtz coil. Measurement of the refractive index of the ferrofluid film under different magnetic field intensities with Michelson interferometer by contrast measuring method, shows that the refractive index changes with the variation of the external magnetic field intensity. This phenomenon is explained by the modification of the size of ferrofluid particles under external magnetic field, which changes the ratio of dimension of the chain of magnetic particles to incident wavelength, thus altering the refractive index of the magnetic fluid. Based on this, the relation between the refractive index and external magnetic field intensity is established, which will provide a new technique for the development of new magneto-optical devices for magnetic field measurement, optical valves and so on.