A new type of symmetrical metalcladding optical waveguide structures is proposed. The guiding layer that supports the oscillating field is submillimeter scale and severs as the sample room. The waterbased Fe3O4 magnetic fluid with a volume fraction of 0.15% is injected into the sample room. The ultrahigh order modes are excited by a 860 nm laser beam on the structure with small incident angle (＜5). Owing to the strong concentration of the magnetic field, the sensitivity of the ultrahighorder modes is very high. The rise and fall time of magnetic fluid is 2 ms when 10 mT magnetic field is applied.