A fiber integrated modulator is proposed based on a new kind of optical fiber with a fiber core on the inner wall. This optical fiber has a pore structure and its core is suspended on the inner wall of the pore. The magnetic fluid containing super paramagnetic Fe3O4 nanoparticles is synthesized, then it is encapsulated in the hollow optical fiber through molten connecting with a multimode optical fiber and used as the cladding of the fiber core. By an evanescent field, the magnetic fluid generates absorption effect on the light and its intensity is controlled by an external magnetic field, so that the light modulation is implemented. Moreover, the light at a wavelength of 632.8 nm is coupled in and out of the modulating element by tapering the coupling point. The experimental results show that only 2.0 × 10-2 μL of the magnetic fluid in the system can significantly impact the optical attenuation under different magnetic field strengths. When the magnetic field strength is 38 914 A/m , the saturation modulation depth is 43 %, and the system response time is below 120 ms. The system can also be used in light switches, optical fiber filters and magnetic sensors.