Current diagnostic techniques do not reliably detect cancer at early stages, and traditional chemotherapy lacks specificity and causes systemic toxicity. To address these issues, multifunctional nanomaterials are becoming more widely studied as a means of cancer detection, therapy, and monitoring. Here, iron oxide (Fe3O4) nanoparticles were conjugated onto the surface of multi-walled carbon nanotubes (MWNTs), which were then modified with polyethylenimine (PEI) and polyethylene glycol (PEG) to improve their solubility and biocompatibility. Finally, human telomerase reverse transcriptase (hTERT) siRNA was loaded on the MWNT surface by electrostatic interaction to obtain a multifunctional delivery system (MWNT-Fe3O4-PEI-PEG/siRNA). This delivery system efficiently delivered siRNA, allowed targeting of certain sites by magnetic fields, facilitated photothermal heating by near infrared irradiation, and enabled magnetic resonance imaging, thereby indicating great potential for cancer theranostic applications.