A type of ultrathick double metal cladding optical waveguide with a submillimeter scale was used to investigate the narrow bandwidth filter of magnetic fluid, in which the magnetic fluid were injected into the sample as a guided layer. While the 1 550 nm laser beam was incident on the waveguide with resonance angle, ultrahigh order modes would be excited; the magnetic fluid was in the oscillating field. Then based on ultrahigh order modes, the refractive index of magnetic fluid was changed owing to the strong concentration of the magnetic field. The characteristics of narrow bandwidth filter were obtained by detecting the reflected beam on waveguide. In the experiment, the wavelength of the incidence beam was to be form 1 549.440 nm to 1 549.585 nm, and the bandwidth filter to be 0.06 nm. Obtained results show that the change of reflected beam is 11.9 dB when the magnetic field changed from 0 to 519 kA/m in the wavelength of 1 549516 nm. Experimental results indicate that the optical filter has narrow waveband and higher sensitivity.