Magnetically driven drug delivery systems of superparamagnetic iron oxide nanoparticles have a considerable potential as candidates to overcome the present obstacles of drug delivery in anti-tumor therapy owing to its remote controllability by external magnetic fields and other unique properties. In this work, a biodegradable anionic copolymer with side carboxylic groups named methoxy-poly (ethylene glycol)-block-poly(α-carboxyl-ε-caprolactone) was synthesized to complex iron oxide magnetic nanoparticles and load paclitaxel (PTX) to form dual-stimuli responsive copolymer-magnetite superparamagnetic nanocomposites with an elastic core and carboxylic groups on the surface in a very easy way. The physiochemical properties of these nanocomposites were measured. High PTX loading content and high saturation magnetization were obtained. Being proved to be stable at a wide pH range and low cytotoxic in vitro, these nanocomposites presented faster PTX release in vitro at pH 6.5 than at pH 7.4 and obviously reduced burst release.