By integrating high-performance CT imaging and photothermal therapy (PTT) into one nanoprobe, an effective theranostic can be achieved for clinical cancer treatment. In this study, the graphene quantum dots (GQDs)-coated bismuth (Bi) nanoparticle (NP) as a theranostic nanoprobe is synthesized and its capabilities for computed tomography (CT) imaging and PTT are investigated. Such nanotheranostic exhibits good physiological dispersity with satisfactory blood compatibility and cytotoxicity. Most importantly, the GQDs-Bi NPs offer strong and steady absorbance profile in NIR region with excellent photostability, which can remarkably convert photo-to-thermal with the photothermal efficiency of 30.0%. Thanks to the powerful PTT effect, co-delivery of GQDs-Bi NPs/NIR laser can effectively induce HeLa cells death in vitro. Cooperatively, NPs hold X-ray attenuation coefficient for high-contrast CT imaging with the corresponding CT improvement efficacy as high as 32.7[Formula: see text]HU[Formula: see text]mg[Formula: see text]. The obtained results highlight the potential of GQDs-Bi NPs as a successful theranostic nanoagent for CT imaging and cancer photothermal therapy.