Carbon nanotubes-coated/filled by Fe3O4 nanoparticles were successfully fabricated by a facile in-situ coprecipitation method at low temperature. The effects of the adding amounts of Carbon nanotubes (CNTs) on the morphology, structure, magnetic and adsorption properties of the nanocomposites were studied in detail. The XRD patterns showed that the sizes of Fe3O4 nanoparticles in the nanocomposites could be well tuned from a few nanometers to a dozen nanometers. The SEM and TEM images revealed that different coverage density and composite mode had happened between CNTs and Fe3O4 nanoparticles. All of the nanocomposites exhibited favorable superparamagnetic properties, and the saturation magnetization was affected by the average grain sizes and the loading amounts of Fe3O4 nanoparticles. Strong magnetization and high dispersion of Fe3O4/CNTs nanocomposites implied the excellent adsorption capacity and recycle of nanocomposites in the practical application. The adsorption kinetics of methylene blue (MB) was well described by the pseudo-second-order kinetic model.