Porous Fe3O4/C composite nanobelts have been successfully synthesized by carbonization of the electrospun polyacrylonitrile/polymethylmethacrylate/ferric acetylacetonate nanobelts. SEM observation reveals that the products are porous nanobelts in morphology. Electrical and magnetic properties analyses show that the porous Fe3O4/C composite nanobelts possess tunable electrical conductivity and magnetic performance. The N-2 adsorption-desorption measurements demonstrate the surface area and the pore size of the porous Fe3O4/C composite nanobelts are enlarged with the increase of Fe3O4 contents. The obtained porous Fe3O4/C composite nanobelts exhibit efficient adsorption property for organic dyes in water and excellent magnetic separation performance. The isotherms and kinetics of adsorption process are determined and analyzed in detail, which are found to obey the Langmuir isotherm model and the pseudo-second-order kinetic model for Rhodamine B, respectively. The excellent adsorption capacity can be attributed to the porous structures, which will make them promising adsorbents for water treatment. This work provides a new insight into the design and development of functional carbon-based nanomaterials.