Titanium dioxide is an attractive photocatalytic material with a wide band gap which can be applied in chemical processes to degrade organic compounds. When TiO2 nanoparticles are dispersed in water solution in suspended state they possess high photocatalytic activity, resulting in effectively degrading the hazardous compounds in wastewater. But when the TiO2 nanoparticles are used in suspension state, it is very difficult to get them back and make them reuse, which would limit their application due to the cost. Here, we report the preparation of reusable TiO2 composite nanoparticles with a core-shell structure, which consist of a nanosized Fe3O4 core and the encapsulated TiO2 shell. The experimental result shows that these nanoparticles can be used in suspended state and exhibit high photocatalytic activity. In addition, the photocatalytic nanoparticles with core-shell structure can be easily withdrawn with a magnetic field.