High-resolution imaging of molecules intrinsically involved in malignancy and metastasis would be of great value for clinical detection and staging of tumors. Based on the selective cell-penetrating ability in the tumor sites of the activatable cell-penetrating peptide (ACPP), the superparamagnetic Fe₃O₄–silica composite nanoparticles (SSCNs) coated with Au nanoshells (AuNSs) were prepared and subsequently surface-modified with polyethylene glycol (PEG) conjugated an ACPP to form SSCNs@AuNSs-PEG-ACPP nanoparticles for realizing tumor site-specific multimode bioimaging and photothermal therapy. SSCNs@AuNSs-PEG-ACPP nanoparticles showed high photothermal efficiency, X-ray attenuation property and T₂ relaxation rate, which were confirmed by a series of characterizations in vitro and in vivo. The enhanced accumulation of SSCNs@AuNSs-PEG-ACPP nanoparticles in the tumor was investigated by the in vivo X-ray computed tomography (CT) imaging and magnetic resonance (MR) imaging. SSCNs@AuNSs-PEG-ACPP nanoparticles will be likely to realize simultaneous CT/MR bioimaging and photothermal therapy on the tumor by the combination of superparamagnetization, X-ray attenuation property, near-infrared (NIR) absorption and tumor-selected accumulation.